HP at RPM is important. The generator will be 1800 or 3600RPM depending on whether it is a 4-pole or 2-pole design. You need to know the HP of the engine at that speed, not rated HP of the engine. You may find that it's less than that of the propane engine that's currently fitted. Onans used air-cooled 2.4L 4 bangers for years that made a whopping 21HP at 1800 RPM. Engine was rated at 52HP, but around 4500RPM.

You need 2HP per KW of generator capacity if you expect to start and pull rated load.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)