There’s one problem with the Zenoah 320. The crankpin/rod big-end bearing. When they put the bigger cylinder on it they increased the size of the journal on the crank. Due to the increased speed (bearing rollers) they tend to skid on the crankpin and gall the journal. That engine will handle up to 10,500 rpm but no faster than that. So the gearing has to be right for it to keep the engine rpm down.
That is why I built stroker 290’s. As Zenoah increased the displacement of the original G23 they did it by progressively increasing the bore size. The increased piston weight gradually caused the vibration to get worse because they use the same crank in all the small blocks up to the G290 - 28mm stroke.
Using the 2mm stroker crank increases the weight of the crank counterweights, and with an appropriately lightened piston you can achieve the same balance as the G230. With an increase in piston displacement to 30.5cc and increase in horsepower to 4.6 (with some mild port work). Also requires decompressing it so it can burn premium pump gas. There’s three ways to decompress it - either use a 1mm shim on the cylinder base gasket, machine the combustion chamber dome, or remove some material from the piston crown (which alters the port timing).
With the 320 they stuck with the same 28mm stroke and increased the bore to 38mm, so balance is going to be a bigger issue than it is with the G290 in its stock form. That was part of the reason for going to more mass in the crank for the 320.
The stroker crank for the Zenoah small-blocks is a special order item that comes from Zenoah/Husqvarna
My answer for bigger more powerful helicopters up to 25-30kg takeoff weight and rotor diameter of 2.5m is to use two G290-based engines instead of a larger single engine. Can get the same power from two Zenoah small-blocks as a turbine at slightly more weight in engines, but radically less fuel weight than a turbine. Two small-block Zenoah’s burn ~52cc/min in a 25kg helicopter compared 190-200cc/min with a turbine.
Incidentally, RC-sized turbine engines have the same problem as electric motors - the bearings in the hot section. Those little ceramic ball bearings that cost $185 apiece only last 25 hours in a JetCat engine. All because the RC-format turbines don’t have pressure-lubricated bearing sumps like a big turbine engine has. Even with expensive ceramic balls and races, without oil to keep the bearing cool, wash out contaminants and lubricate it they have a limited life. And then the engine won’t start, or worse yet it hung-starts and you end up burning up the turbine wheel in the gas generator and you’re looking at a $1,000 engine overhaul.