I was having trouble with it turning over. I replaced the battery but all that changed was that it spun the starter motor which failed to engage and start the engine. I thought the starter motor was bad and replaced it. The same thing happened. It failed to engage and start the engine. What do I check next?

Did you replace the bendix when you replaced the starter or dd you use the old one. The bendix is the gear on a candy cane shaft out the front of the starter. When you engage the starter the shaft spins and makes the gear slide forward and engages with the flywheel. If the gear does move forward the starter will just spin at a higher than normal RPM with no engine turn over