The G10 engine can be leaned over up to 80deg towards the exhaust side to raise the thrust line in tractor situations. A small turbo can be fitted to increase the low end torque, a muffler is not required when a turbo is fitted so there would be no net weight gain.
Many planes are designed around the engine, so if you want to use something else, it might cut into the design if you don't think it out. Not as plug and play for numerous reasons. A good ole Rotax s hard to beat if in good shape but the 1/2 VW would be my pick.
There are 4 stroke paramotor engines which are pretty light.
What sort of ultralight are we talking about? I'm assuming a part 103 legal ultralight. Even then, ultralights with long wings, clean lines and light pilots will need less power than others. I'm thinking one of the latter might possibly be flown with a vertical shaft, 500 cc Briggs that's got a weird redrive or has been converted to horizontal. But, of course, that's a project.
The definition of an Ultralight is different in different countries, if you strip the Suzuki down to the bare engine, add a side draft 32mm Bing and a simple 3 into one exhaust the actual weight is about the same as a 582 Rotax, in direct drive configuration it would be only about 40ish HP but would be easily light enough for our Ultralight Category in OZ.