the first thing I would check are the exhaust pipes. They come out of the front of the engine, and would be the first thing to be crushed in an accident. Look for dents. Since it runs, start it up and spray the connection between the headers and cylinders with carb cleaner. If the engine dies or changes rpm, you have an air leak. Try this between the cylinder and the cylinder head too. Do this to all of the connections.
If none of those are your problem, try removing one of the spark plugs and checking compression. Put your thumb over the spark plug hole and check to make sure it pushes your thumb off. Repeat that on the other cylinder. If this doesn't check out, the whole engine needs to be taken apart and rebuilt.
Check for spark by removing the spark plug, popping it into the plug cap and setting it on top of the engine (touching the metal) make sure both cylinders have spark when it turns over. If not, trace your problems back through the rest of the ignition.
If that isn't your problem, try pouring a little bit of gas into each cylinder and try to start it. If it runs better, clean your carbs, because you have a fuel delivery problem.
If none of that works, check the ignition timing. You might have sheared the woodruff key on the flywheel.