I would go digging for STC's (supplemental type certifications) where somebody may have put newer engines on the old contraption already.
E.G. As far as I know Jet Aviation used to fly a DC8 or 707 with new engines. Needs verification.
Many old piston engined aircraft have been modified for turboprops DC3 (not sure) and the nice tri motor conversion of the vintage Dornier.
I know the problem is inherent on the C4xx series of light twins. The centurion looked as an option but then ...
Finally all a question of money. The manufacturers sure are happy to supply as long as it pays for them to have the old parts.