RicardoM

Here are some illustrations that show how the fresh air / recirculating flaps work.
 

 

As you can see, the sector gears you asked about are part of the recirculation flaps mechanism. Yours is blocked. By unblocking it, the recirculation button should restore its function. Of course this is true if the vacuum damper and the vacuum circuit that operates the flaps are intact.
 
Re. relay J229. See electrical diagram below. Compare the wiring and restore it to original. Previous owners of the car tampered with it.
 

 

https://www.drive2.ru/l/6586962/

The markings for ignition timing are on the cover of the distribution chain around the crankshaft pulley. See last photo for correct timing (-2 +/- 2). But if idle is poor I doubt is from timing unless you messed with it lately.
 

 

 

An engine works most efficient when the coolant temperature is at around 90 C. The thermostat is a temperature regulator that has two phases of operation:
It helps heating up the engine fast from cold start by staying closed and allowing the coolant to flow only in "the small cooling circuit" cools the engine when too hot by opening "the big cooling circuit" Running without a thermostat prolongs phase 1 and the fuel consumption is higher.
 

 

If the thermostat is the culprit indeed (so far your observations point to it), you can take it out completely as a temporary solution. The only drawback will be a higher fuel consumption because the engine will warm up slower.
 
A slightly open stuck thermostat could explain both the high duration for warmup from cold and the overheating on highway.

The first rule when getting a new part says "compare side by side the old and the new part".
 
I only have this for you. HTH.
 

The circlip is used on VW Caddy pickup too.

Could any Skoda Felicia owner use etka.cz VIN decoder with positive results?
 
I only get this error:
 
Something is wrong…
There is no result for your VIN code or your entered VIN code is incorrect!
Please try again!

The main difference in wiring between MK1 and MK2 is that pin 7 is connected or not to ground. The photo below should help you look for differences between old board and new board. If they are identical on the tracks I've highlighted, the only thing you have to look is how pin 7 of black connector (T10d) gets wired.
 

Do you really expect that resistance still has the correct value after it got so hot that almost self-destructed ??! I mean... the PCB got charred, cracked, and punctured...
 
I've uploaded relevant photos. Hopefully you will believe what I'm writing. By the way, I am an electronics engineer.
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¿De verdad esperas que la resistencia todavía tiene el valor correcto después de que se puso tan caliente que casi se autodestruyó ??! Es decir ... el PCB ha sido carbonizado y perforado ...
 
He subido fotos pertinentes. Espero que ahora vas a creer lo que estoy escribiendo. A propósito, soy ingeniero en Electrónica.
 
 

 

 

The first thing that came to my mind was "define GOOD"...
The only 'good' thing about current situation is the HG failure is minor. Regardless of what failed where, the head must come off. Oil in coolant or coolant in oil indicate a head crack or a head gasket failure. When you'll open the patient for surgery, try not to disturb the head gasket too much if you want to 'read' what area failed (both sides).

You need a new window winding mechanism or at least a working second hand one.
Look for absence of corrosion and cracks. Old plastic gets very brittle.

If you plan to rebuild the engine, consider taking us on board if you have a friend that could document the process while you are using the spanners.

TeflonTom gave you a very good answer. He has a Felicia Diesel too and he knows those engines inside out.
 
The photo is just an example for reverse engineering the PCB. Skoda fitted 12 (twelve!) types of dashboards on Felicia depending on engine type (135, 135/6B, 135/6M, AEE, AEF), year of manufacture ('94-'96, '96-'98, '98-'01) and extra-options (LX, GLX). The photo happens to be from a '96 136B GLX Felicia. Your dashboard will be slightly different, but nevertheless the electronic diagnosis is the same. Basically rpm impulses from ECU (135/6B, 135/6M, and AEE engines), alternator (AEF engines) or ignition coil (135 engines) get at pin 7 of PCB yellow connector, then the impulses are reduced in amplitude to about 3 Volt pp and they feed the input (pin 13) of a custom IC made by ITT (KEFB-842) that drives the gauge as in the image below.
 

My ECT has 2007 Ohm at 20C and 197 Ohm at 90C. It is easier to calculate what resistor to put in parallel than to order the correct sensor...

Park the car slightly uphill or raise front axle about 20 cm (8 inches).
Remove coolant temperature sensor.
Start filling the expansion tank with coolant.
When coolant with no bubbles starts to flow through the hole, install the temperature sensor.
Place the cap on expansion tank.
Close the bonnet.
Start the engine and let it run at idle till the radiator fan kicks in.
Stop the engine and let it cool down completely.
Add coolant in the expansion tank till half way between MIN and MAX marks.

With engine stopped, if the gears shifter feels the same when you try to engage gears, it's a clutch disk failure, not the gearbox.

There you go. The connector with rust is a common problem. Water seeps in, pins oxidize, the high contact resistance and high current produce heat that melts the wires and connector together. Worst case you'll need new male connector and new block of resistors.
Repair photos below. But use the same wire colours and keep track which goes where.

See attached in vectorial format.
Preview below.