A 160 cc full crank motor with the muffler on the side would bolt straight on ( example ,Mayfair mower engine)
If you use a motor with the muffler at the front you will need to re drill the 4 engine mounting bolt holes in the base and cut a slot in the front of the base for the muffler.( as you say you don't want to modify the chassis)
The front muffler motor can be turned to fit your base but you would need to use the Victa 1968 New Utility cowl starter and fuel tank .The problem with doing this is there's no vacuum connection on the Victa 1968 New Utility cowl for the carby governors diaphragm.( governor won't work) it's less work to buy a mower with the muffler at the side for a donor engine. Another problem with your cowl on a later engine with a decompressor is that the cowl would need modifying to fit over the decompressor unit.
Any Victa full crank motor with the G4 carby will fit but the side muffler motor is less work.
I'd swap to the later motor rather than swap the carby as it's less work and Victa 2 stroke mowers are cheap. I've also seen a few cheap later model Victa Utilities for $10 to $30 on eBay etc.
The Victa Utility below has been re drilled and has a new hole in the front of the base for the muffler.( engine swap) 2nd last mower is the $20 Mayfair and the last is a front muffler $30 mower. Cheers Max.
