Hi guys, I would like to hear your opinions and suggestions.

If I buy a bone stock Volga (GAZ 24), first generation with a 2.5l 95hp inline four engine, and a 4 speed manual and I want to make it super reliable (which it kind of already is) and a slight bit faster while also making it a bit more economical.
Should I swap it with a 3.0l Mercedes-Benz OM603 atmospheric diesel engine from a W124 300D and whatever (manual) gearbox comes with it, or should I keep the same engine?