Big bore kits are available from Kientech, ProCycle, and probably others. Below are horsepower and torque curves for standard DR350, 440cc big bore, and 480cc. The standard displacement has a TM33 pumper carburetor and an open air box. The 480 has a 92mm piston, TM33 carb, Pro Circuit pipe and an old White Brothers cam.
Horsepower curve
Torque curve