Onan Marquis 7000 Generator Starter Revs But No Action

After a few times putting the new starter in and out of the genny and then confirming it works on a bench test we concluded the starter is engaging but not able to turn the flywheel. When I try to turn the flywheel with my fingers inside the starter hole it is very difficult to spin in the direction the starter is trying to move it. You can't access the flywheel with the top cover off from what I can tell. Is a rebuild or a new generator the next step?