Does anyone know if a mk4 Golf 1.9 sdi gerbox will fit a mk1 Fabia SDI 'ASY' engine?

It's highly likely, yes. I'd certainly give it a try since it's pretty simple to remove the Fabia box if you have access to a four poster.

I might be wrong, but I thought the speedo pickup and gearbox mounting brackets are different on the Golf compared to the Fabia/Polo.

 

Okay its a different gearbox but I know the Golf 1.8T gearbox does not like the Polo 1.8T Gearbox mount because the Golf Gearbox casing has a wider casting. But who knows, the 1.9 SDI boxes might be all identical across Golf/Fabia

 

I hope i am wrong, but as Sepulchrave said, give it a go!

Got a feeling the Golf like the Octavia is an 02K gearbox where the Fabia is an 02R.

well, turns out the gearboxes are completely different, the linkage on the gold is rod whereas the fabia is cable, also the clutch cylinder is different.

so after coming to a dead end with the sdi, 'cant find a gearbox for a reasonable price locally, ive decided to go a different route and buy a TDI donor car and do a engine swap, as works out better value as sdi boxes always seem to fail and not spending £300 on a gearbox that'll blow up in 2000 miles 

Is your car in that good condition that an engine swap is worth it?