Well there are a bunch of reliable late 90s and early 2000s German engines that would make that thing ridiculously fast compared to now, pollute less, burn less fuel, and would be pretty easy to maintain.
Long as you avoid all the ones with known pitfalls and research standalone ECU options first of course.
I’m partial to Mercedes engineering myself, I’d tell you to use an OM646. But there’s nothing wrong with an M47 or a VW 1.9 tdi either. The PD version of the tdi is slightly more complex than the oldschool versions (66 and 81 kW), but would get you ridiculous performance and fuel economy considering how little your car weighs.
Of course if you had more space in there, I’d suggest an OM648 or M57, but I don’t think you’d get an inline 6 to fit. MAYBE an OM647 since it’s an inline 5?