The weight of an engine and all that has to come around is not only connected to the engine or to the 4-stroke type. The number of cylinders and the complexity of the engine counts more.
My little 4-stroke engines are very simple, no need to add a lot of parts needed for its operation around. A sheet metal duct does well as an exhaust system given the low engine revolution. A 2-stroke needs a large calibrated exhaust system, if you keep only the tune pipe, it's tiring not bad. I did not even add the weight required at the front of the chassis to cover the bulky exhaust system. A 550 needs more than 6 liters of gas to drive at least 4 hours continuous, no choice to consider this factor, unless you turn around the cottage, the gas can already ready, are not worth closing the plug.
In the end, the only one that is comparable in weight is the 277 Tundra (possible to mount the HP, but then no torque and very noisy) And if you want to get your hands on one of these engines (especially with RER) You have to buy a complete Tundra, it's far too rare a complete engine alone.
The engine MEGA (Australia) and Le Panthera (Quebec) are 2 beautiful exotic engines big single cylinder 2t. They are very expensive and not certain of their reliability, but an unlimited budget can be.
The MEGA has no 12volt output, (no light) the output of the crank is too small to put a CVT clutch.
With the Pantera (engine designed to fitter in a bike CRF450 or MTB TRX450R), I would need a training system more solid (heavier) to support the power (not added in the table) he would need to a new base to fit a CVT clutch, which would remove gears (and weight), would compensate for the added strength of the drive system. But it's a crazy project $$
Rendered that weight, as well put a Rotax 600 first generation (110hp) as in my MXZ, which was much lighter than a 600Etec (800 base and full of components). The 600Rtec consumes just a little less than the 600 carb of the years 2010+. The 600CARB can work with gas mixer, not the Etech which needs a tank of eight and all the hardware.
New technologies have a weight and a price, they invent a lot of things to remove weight, but it does not compensate for all the components that are added.
With a 600, That would give a Small-Doo whose weight is getting dangerously close to my lightening MXZ, an anvil of 390lbs dry. It's 90lbs of difference dry for dry in addition (30% more weight on the skis) Of course that the report power weight is higher with almost 3 times more HP, but the flotability, the capacity to stop and to leave again anywhere is not even comparable.