As a guy who has sold dozens of these kits, you want to buy the kits manufactured at the YD (Yuan Dong) or NTJL (Nantong Jiali) factories. Currently, Grubee kits are the only importers of these kits. Rico Group imports the "BoyGoFast" engines. And the RAW engines come from the Jiangdu Flying Horse factory.
They are all pretty much the same, but the Grubees are a bit better with regards to care in assembly. The BoyGoFast 50cc slant heads with steel cylinder liner are pretty good engines, but the Grubee Gen II's have a superior installation kit.
I'm about 175 lbs, and the slant head 50cc engines are good for 30+mph - but for a 210 lb person, maybe a 66cc kit or a larger sprocket on a 50cc kit is the ticket.