After seeing about 100 stock puch crank bushings and finding 1/2 of them bad to begin with and kitting dozens of puchs with e50s and za50s I keep the hi rpm set ups to needle cranks and even if your 30 year old bushed crank is perfect condition it will fail/melt with prolong hi rpm use .
I just finished rebuilding a e50 for a young guy who just wasted his crank. He ran a boss pipe, 15.15 sha, 50cc tccd Kit and 16x42 gears. The carnage includes a melted bushing and a damage at the piston pin that required a new piston. Lucky he shut it down when it made clatter noise
I sold him the e50 last year and it only had about 1200 miles on it and the top end and bushing were mint . I helped him with his kit, clutch, timing and jetting. He ran a beat Proma circuit that I sold him and 16x38 gears. After he broke his Proma off he got a boss then he had to down gear to 16x42 gears stay on the pipe and he actually went faster near 50mph.
About a week later the bushing melt like I said And though I'm cheep it still cost him almost 300.00 with crank, bearings, seals, gaskets and piston.
Run circuit/estroil pipes, run medium gears on your stock crank and enjoy riding this summer