Mav27
The transmission is the limiting factor on a 369HS or OH6. On an ICAO std day of 15 degrees the engine will provide you a tad more power than the trx can handle. However as the temp goes up or you go higher you will run out of tot before torque hence the more powerful C20 ( yes you can put a C20B) in the C model. However the limiting factor is still the drivetrain. Thats why D model came along, bigger trx extra blade etc etc
The C20 series has the follwing times for turbine
N1 side 1750 hours or 3000 starts
N2 4500 hours or 6000 starts
C18 has 1250 hours N1 and no start limitation, sorry cant remember N2
The other way to go is to buy the machien and when engine needs overhauling slap in a C20 adds value to machine. Have to check have correct trx, tailboom and tr blades though