Re: what are these engines made of?
Lot of good advice given in previous posts.I suggest preventing future damage to you case and to your person. Chain failure isn't uncommon on two wheels for whatever reason breakage, link keeper loss, loose chain etc. First already suggested in previous post, proper chain tension, 2nd. lube well and often, 3rd. install link keeper correctly with closed end traveling in the same direction as the wheel, 4th. use the correct chain and buy quality. Last install a case saver & possibly your life saver as well. These (when properly installed) prevent chain to case lockup by threading the entire length of chain over the engine drive sprocket without binding into the engine case. Cost between $10. & $60. on E-bay & their easy to install using a hand drill & basic tools.
I'm not a vendor though I show an example of one type mounted on my Indian Navaho bike. It's the more expensive type about $60. but I use a simpler version that should work just as well on another bike $10. or so.
Unfortunately I lost a chain at about 25mph (keeper fell off, my fault) on the bike pictured with chain saver installed & never felt it, engine just kept reving as I slowed to a halt with the entire chain on the ground aboubt 30 yards behind me! Quite often the rider is ejected over the bars when chain to case lockup occurs at speed & both rider and engine are damaged. The case saver prevents both injury and engine damage. Some will advise otherwise to initially save a few bucks. 50 plus years riding motorcycles has convinced me to exercise caution. They might also think running non-reinforced fenders a good idea also, but one trip over the bars will convince them otherwise...if they survive. Rick C.