my 66cc GT4 Starfire's came with that big motorcycle chain, which is almost
a #41 chain. i've snapped every one of them. the last one came back and smacked into the case, breaking a mounting stud off and cracking the left side case.
i've since upgraded all my chains to Diamond #41 chains, made in USA. got 'em off ebay for cheap.
you can buy chains from fastenal, tractor supply, and other industrial supply places, but my advice is pay the extra for american-made quality. i bought a 10' china chain from fastenal, and broke that, too.
my motors are modded and have a lot more torque, so that helps to destroy cheap chains, but i'd still replace 'em even if i left the bike stock.
for a chain breaker, Harbor Freight has a cheap one that works great for larger chains. mine came with 2 different size pins, one smaller that fit bicycles, but it seems none of them come with those anymore.
here's the link: http://motorbicycling.com/f34/chain-...ght-12689.html