This. If you are mechanically incline. Yank the motor and take it to a B&S small engine repair guy and have him tear it down and estimate of rebuild. If not, take the whole mower in.Can't you have the motor rebuilt by a small engine guy? These aren't exactly F1 engines.
If the cost of repair is more than 50% of a new motor, buy an new exact replacement, install it and call it good. Otherwise, authorize the rebuild.
BTW, I think I’ll be checking my mowers oil filter before reach use.