Get an ST170 Focus engine which is basically a Zetec.
Sell the bike carbs and buy Bike Throttle Bodies, they will need spacing but they then sleeve over the standard Ford inlet manifold once the plenum has been removed and use them to control the air then use the standard Ford injectors and fuel rail etc and away you go.
You will need an aftermarket ecu
You'll need a 1.8 Zetec flywheel, re-drilled to take a Pinto Clutch and these are available off the shelf from Retro Ford in Corby (01536 7479780)
170bhp of reliable power.
Add a vernier pulley on the exhaust cam and they are 180bhp
I4's from a MkV RS2000 or the 2.3 from the Galaxy are pants.
They weigh a tonne.. just like an old pintosaurus and they are a nightmare to tune and get decent power out of them cheaply.
Unlike a Zetec.
Put it like this, the black top in my Anglia is running a pair of Kent Cams and a pair of 45 carbs with some head porting and one of our 4 into 1 manifolds and its 187bhp
which is pretty respectable.