To answer you questions we really need to know WHICH 18.5 B&S you have. Model number, as there are several that fit that description and some are radically different.

IF this happens to be an OHV single cylinder, I have detailed instructions for adjusting the valves which is likely the problem. Address below, put in proper format and remind me.

IF it is a horizontally opposed twin, I have a list of possible causes for hard cranking.

Either way, you have likely fried the starter by holding it engaged too long trying to make it turn the engine. Brief engagement followed by a considerable longer cooling period must be used. B&S starters are highly interchangeable so a good used one should be fairly easy to find.

Walt Conner
wconner5 at frontier dot com