decomp set ups are readily available, if you ran a yammi WR500 motor,they are torque monsters you get a wide ratio gearbox and you can ditch the cooling system to save even more weight, only the last ones were water cooled, had a short product run and pretty rare...but they do exist if you want to keep it all yammi
parts wi$e it might be more viable to run a more popular engine, like KX or CR, prolly safer in slower moving airflow situations anywaythan an air cooled 2s, you get a much bigger window of time before the engine get pissed off and locks up
KTM 380`s are also a VERY stout motor, in the right hands just as fast as a half liter, they also run the same bottom end (cases,box, not sure about the crank) as the 300, 330 and 360...they also made a 550 but those fuckers are left hand kick
there is a guy that put a 380 2s into a 450 frame for MX on youtube...and it is a fucking weapon
so if your going to build a killer XC hybrid bike, you got a fukton of options...gl