50 tooth is too slow and not worth the increase in stump pulling up steep hills at the expense of top speed.
A 48 tooth sprocket is your best bet. I live in Colorado at 7000 feet altitude. Every combustion engine (piston, jet, turbine) loses 3% horsepower for every 1000 foot increae in altitude. At my altitude, I'm losing 21% of the engine's power. I have found that a 48 tooth sprocket is all I need to climb even the steepest COLORADO hill with ease. I haven't found a hill yet that can't be tackled with a 48 tooth sprocket. A 50 is JUST TOO DARN slow for an 80cc engine that is slow revving to beign with.
BTW, I have climbed mountains with my bike and my 48 tooth sprocket took me all the way up to 12,000 feet in altitude with no problem.