I am trying to figure if the picture shows that the angle of the drive sprocket (rear wheel) and the sprocket that is by the engine/clutch (whatchmacallit) output sprocket had before the chain came off the front sprocket and also broke stuff, was the chain at a slight angle.
I can see that the engine platform has I think some side to side adjustment.
Also I know that some people, especially on DIY builds, have widened the rear fork and spaced the rear wheel more over to the left to get the chain in as straight as possible a line front to back.
My current build I have spread the rear fork and spaced it. It is under a bit of tension, so cutting and re-welding the left side of the rear fork I may have to do. I have also heard of flame heating so it won't crack the tube when bending the rear fork on the left side outward a bit.
I have wondered if tempering again by quenching in water to make it less flimsy after heating is necessary. During welding I have not heard of this except where someone, not for on motor bikes, had welded to a torsion bar and there was talk of needing to re-temper it afterwards.
Anyway hope you can get parts or have a repair done by welding whatever is necessary. Say, once you get it repaired, if you can turn engine output either manually by hand or slow throttle running with the bike on a test stand to see how it operates before under full riding load. This way you can see if it is going to not have problems and need some adjusting, I would advise doing that.
Forgot that if you can turn the rear wheel and have it turn the sproket forward on the engine, that is with the engine off (not running) and by the same token check how well the chain moves. If it tries to jump off or catch any teeth while turning.
Safely use goggles, leather apron, no loose clothing, or whatever necessary to accomplish this if you do try as I mentioned! I am going to soon be trying this week or next, I'll let you know.
Measure Twice