Hi, used to own a Puma.

The 1.7 Puma engine has a VVT (variable Valve timing) system. Yes, It's common conversion with the Fiesta, as its basically a Fiesta Coupe.
1.7 Puma engine will be a lot more expensive to look after, for example when I had my Puma it cost £425 to get the cambelt changed (and it has to be done at 8yrs) not 10 years like the Focus belt.

I think you'll have a few problems TBH, you may as well save up the pennies and buy yourself either a MK1 2.0L Focus ESP or ST170 if you can live with the insurance costs.

Its a nice idea though!!