The current gas gensets from Onan will easily fit the space where your older Marquis was fitted. Length and width same, newer gensets about 1 inch taller. All the hook-ups are on left side of new sets, so you may have to lengthen batt cables, fuel line, a.c. wire harness etc. Onan offers 5500 and 7000 watt models; Model 5.5HGJAB, and 7HGJAB. Onan also has EFI units, same size dimensions, but higher priced, and will have to install high pressure fuel pump for EFI units.
I have done this swap numerous times, pretty easy. The newer sets are quieter, as they have full sound enclosure, but I really like older sets if they are repairable, as they run at lower speed, have bigger oil sump, and gen is direct drive from engine, versus belt drive on newer sets.