With the question regarding checking chains; if the chain feels like it is hanging up on the teeth of the sprockets while riding the bike, the chain may not last long. It is possible for a sprocket tooth to cut through and break one side of a link and the chain still be usable for a while. It would take a close inspection to see this problem. The main problem with chains is the clip holding the master link together falling off or a half link pin failing. Chains also stretch as they wear and no longer fit the sprockets properly causing the chain to hang up on the teath. Chain failure, especially when you attempt to push a chain a lot of miles can be unpredictable. This is where you are better off having a multi-speed bike in the event you have pedal a long distance without the motor. And a lighted bike. The best assurance the chain is good to go is keeping the chain working smoothly with the sprockets.
Regarding the second question; you raised a significant question. Wheels may be subject to wear on the side of the rims from C brakes, especially if they are alloy aluminum, but I am not aware of rim cracking being common, even if ridden a lot of hours. I have been aware of concern in the past about motorizing bikes with front disk brakes, aluminum forks in particular, that the forks could break off when the brake is applied. Some models may be up to the added strain of stopping with the added weight and added speed and some may not. There may not be a way of checking a bike to see of there is a risk of the front forks failing. Where the two cycle engine attaches to the bike frame; the most likely place for the frame to crack is under the front and rear mounts. This may not be visible without taking the motor off, so there may not be a before ride walk around good to go check of the frame. I don't know if is likely to fail without warning, and it may take a lot of riding hours. If it happens, the rider could suffer serious leg injury from the drive chain pulling the engine sideways out of the frame. This level of frame failure has been posted on the forum site, but I am not aware of rider injury occurring from frame failure.
I was talking recently with a relative retired from Outboard Marine who worked for years on the manufacture of outboard motors about the aluminum used the the casting of the parts. The aluminum used was not a alloy. It was observed that by adding recycled aluminum to freshly mined aluminum, the castings were harder and threads would tool better. Just from that I assume that the aluminum used in bike frames, in particular to facilitate welding, is plain aluminum, not a stronger alloy as expected in the riveted together airplanes. From that alone, there needs to be added concern from the motorized bike community as to the effects from vibration; in particular with aluminum frames. Aluminum rims seem to be a different animal and could be an alloy; as are the masts in sail boats.
From my own experience over the years it appears to me a close comparison to the effect of engine vibration on bike frames is found in older back end and underlane bowling center equipment. This equipment is subject to high hours of low level vibration from motors, balls and pins. Examples can be seen of how vibration travels through the tubular and stamped welded together steel until it reaches a solid spot where the vibration is forced to stop, creating a risk of cracking. It may be an exaggeration to say stamped steel subject to low levels of vibration over a long period of time will eventually shatter like glass, but not by much. Motorcycles have much heavier frames than bicycles, in my opinion, as a way of adding resistance to this vibration and its risk of cracking. Making lighter weight bike frames work with gas engines could continue to be a long term learning curve.
