Looking to replace the standard boost pipes as I think they're leaking a tad.

Purchased a: Forge Alloy Bayonet Style Hose Coupler which will go into the EGR bit. Type B 64/54 mm

But what other piping do I need to connect the rest to the Seat Sport Intercooler?

I can't find a 90 degree reducer from 54mm to 51mm...

I'd like to keep the engine cover - are there any out of the box solutions without going custom?

Sfs do a top pipe that replaces the standard plastic pipe to egr but they are mightily expensive!

ASH Hoses on ebay do a 57-51mm reducer - it should clamp down ok if you use some proper tape to build it up a bit. Mine looks like this:

All sourced from ASH Hoses. All in cost about £50 which is much cheaper than off the shelf.

I used a 90 degree reducer (I have an egr delete though)

Then a 45 degree 51mm stainless pipe

Then a 45 degree 51mm silicone pipe onto the top of the Forge kit from the FMIC

Hope this helps

Just noticed I ordered the wrong one, should have ordered type D, 64 to 60mm

Then ordered:

Silicone Hose 90 Degree Elbow Reducer - ID 060>051mm

Alloy 45 Degree Elbow Hose - Beaded - OD 51mm

Silicone Hose Elbow 45 Degree - ID 51mm

Alloy Hose Joiner - Connector - OD 51mm

Hope that lot should fit and be able to get the engine cover on?

Should fit mate, my engine cover fit over that pic above, although its an Ibiza so I'm guessing it'll be a different cover