Couple of conversions done in Germany and Holland, and one in Uk that i know of. Not a popular conversion as Haysesy has said, too much to do and also heavy engine. Can be done of course. You start to lose the lightweightyness point of the polo. Boat + Anchor! Really need to be a short block engine to use in a polo as opposed to tall block and even taller block (ABF) with the Golfs. It can be made to handle, GVK proved this with his mk2 golf VR6 track car.
If staying NA, recommend a 1400 16v AFH or a 1600 16v Polo Gti engine. Polo Gti engine is iron block, lupo GTi is aluminium but issue with starter motor bolt locations IIRC, maybe wrong. Plenty of info on ClubPolo. i believe the AFh 1400 head will fit directly to the Polo GTi 1600 block - better flowing head and not roller rockers.
FI wise - stick a G40 engine in.