Some info that might be useful to the OP, mostly from Wikipedia so may not be perfect
Perkins Prima MDI 1994cc
1990-1992 Maestro D 62 hp
1988-1994 Montego TD 81 hp/ 116 lb ft- no intercooler, EGR
1992-1994 Maestro TD 81 hp/ 4500 rpm, 116 lb ft/ 2500 rpm- no intercooler, EGR
L series TDI 1994cc 1995 onwards
Fitted in the following models in various states of tune:
1995-1999 Rover 220/ 420 D (Turbo) 86 hp/ 125 lb ft non-intercooled
1995-1999 Rover 220/ 420/ 620 DI (Turbo) 105 hp/ 155 lb ft
[Following are all intercooled]
2000-2005 Rover 25 and 45 TD 101 hp/177 lb ft, 113 hp/ 192 lb ft
2003-2005 MG ZS and ZR TD 101 hp/ 177 lb ft, 113 hp/ 192 lb ft
2002 MG ZS TDI 100 hp/ 180 lb ft, 117 hp/ 192 lb ft
2001-2002 MG ZR TDI 100 hp/ 180 lb ft, 117 hp/ 192 lb ft
No idea what the difference was between the TDI/ TD models to give the slightly different figures- could just be Wikipedia haha.
I would go for the most modern engine you can get as they are more economical, have lower emissions, have a lot more torque and are smoother and quieter, though of course the electronics etc will be more complex the more modern the engine.
The cheapest/ most available second hand engine would probably be a Rover 220/ 420 engine; the electronics probably wouldn’t be too sophisticated either. Would hopefully fit the Marina with some work to change the engine from a transverse set up and being distantly related to the O series, it should have the same bolt pattern for the gearbox, possibly. If you fitted the intercooled 105 hp/ 155 lb ft engine the marina would probably have a reasonable turn of speed, or forget the fuel economy and get a 2 litre petrol T16 turbo
Also a period review of the maestro TD
http://www.theaa.com/staticdocs/pdf/...SEL_R9334A.PDF