easiest and cheapest swap would be a 1.3 with a 5 speed out of a mk3 polo. "big block" engines like the mk1 GTI lump need a special subframe, google "Salzmann subframe", I believe they're very expensive and might not be worth the hassle. I also hear that the "big block" motor in the polo makes them nose-heavy due to the extra weight of the engine..
look out for a complete mk3 polo going cheap and buy the whole thing, that way you'll have everything you need to hand. If you end up finding an engine and box on their own, make sure you get the 5-speed anti-roll bar and gear linkage too, they both bolt-up but are different to the 4-speed items. You'll need the gearbox mount from the 5-speed too. The other two mounts are the same for all engines.
I have exactly that in my mk1 Derby (same as polo), the 1.3 engine is fuel injected so I used the whole loom, tank and fuel lines from the mk3 but if you find a carbureted engine it might be easier on the wiring front. A GT engine would be a nice swap too if you can find one that hasn't been ragged...
Cliff or GSBellew are the local Polo experts