Arwyn has a very good point - it depends how much extra 'oomph' you're looking for?
I've got the Kent Crossflow in mine - but have done some mild 'bolt on' tuning - lightened flywheel, large bore manifold and exhaust, electronic ignition with drycell coil, electric fan, silicon leads, iridium plugs, K&N Filter, electric fuel pump. May have added a couple of bhp, or at least restored some of those lost over time.
However, to get any large improvements would mean some fairly major (and expensive) engine work - and I would probably get the same gains with a Zetec conversion for a fraction of the time/cost.
