Marcos Maniacs

I recently purchased a 1970 marcos 3 ltr steel chassis which is in 
need of some mechanical TLC.  It runs, but very roughly.  My 
preliminary research indicates that it may cost as much to 
restore/rebuild the volvo unit as it would to update/swap in a newer 
engine.  Has anyone in the US put a newer engine/drivetrain into their 
marcos with success and could offer advice?  What US spec engines fit 
the marcos chassis? Advice/Info is greatly appreciated.