Troubleshooting is the key to all problems. Learning from others and getting innovative is necessary. In this case Jerry has indicated it could be a "timing" problem and/or worn parts. Now your job is to test in such a way as to prove/disprove/discover what the cause is. There are options to trouble shooting: Replacement, temporary elimination of a feature, testing, careful observation, to name a few. Keeping Jerry's comments in mind, First examine the operation with battery disconnected, good flash light here. If looks OK, try to do the timing "by hand" using all the parts as they are now installed. Somehow kick the pinion in with the linkage and then apply power. Maybe disconnect batt hot, someone pushes foot down, then temporarily touch power back on batt. This may not be the best way to check but it gives you the gist of what I'm saying about trouble shooting. You will have forced the timing to be correct and observe several times to hear and see any change. The person inside will have to be savvy enough to let up when it starts. You will then be able to see that linkage and pinion do in fact work, you may see that it works %100. Or will see it's not timing but tooth slip. In my opinion, if it needs a flywheel, I would replace the whole flywheel rather than a ring gear. Either is a big heavy job.
