There is a wealth of information on this forum if you search for it going back years and years on pro's and con's of various conversions available. From simple 1242cc (1.2) 8V conversions to more complex but still straight forward 1.4 16V conversions to unconventional 2L Vauxhall red top conversion. What route you take will depend on your own abilities, timeframe and budget.
Though if you are going to carry out an engine conversion then it opens up the possibilities of buying a car with an engine issue like blown head gasket cheap. They are out there. I'd be looking around for the cleanest car bodywork wise so you can concentrate on the conversion itself.