gera, I see you're using T8F drive like me. I have the same 11t/72t combination, but with 5:1 gearbox. Your gear ratio is 19.64. My first and second gears are 46.4:1 and 38.2:1, respectively. final drives are 15:1 or 17.7:1.
Still fiddling with the engine clutch, huh? I used pocket bike clutches on my GP460 engines. They bolt right on, IF your engine has a tapered crank snout. I had to shave down the center mounting nut a bit. If running 460 clutch on pocket bike engine, ya might just have to install a washer under the center nut. Honda X50 might also use the same clutch as GP460.
Ya might want to consider using a more dependable engine. The only thing I had to do on my Tanaka 47R engine is adjust the carb and fiddle with the pull start. I have never had any clutch problems with the 47R, unlike my GP460 engine's appetite for clutch springs.
Have fun. That's the main thing.