You can easily convert your 50cc to 80 or 100 w/ a cylinder kit/ gasket set/ carb/pipe for 100.00 bucks (200.00 w/ pipe), and have the same power. Mine is going strong after 4 years, and one new crank.
It's an easy build, and I'd be happy to go thru all the little tweaks that make it last.
B&L's suggestion of a 125-150 proper is the other route. There are weld on "kits" to adapt these engines to your frame for around 75 bucks.
The cool way to do it is buy a crapper 150, and cut/weld the frames together to get a short/low/long 150cc. maybe 250.00 into it. (you need a TIG welder. They are chromalloy frames.