1.2 8v straight swap.
Whatever engine you choose, you need to factor in costs such as insurance premium and even more expensive - the full servicing of the engine.
I bought a 1.2 8v and did following before fitting:
866 cam
Headskim and reshim
New inlet gasket
New exhaust gasket
New headgasket and bolts
New sump
New oil filter, air filter and fuel filter
New alt and timing belts
New clutch
New fluids too obviously.
This is before any upgrades such as brakes, exhaust, inlet mods, induction kit to make the most of it.
I personally wouldn't fit an unserviced engine! I think these are the costs that make the conversion a little more pricy.