I have a 6 hp Merc. 4 stroke, 68 lbs hanging off my pushpit rail along with davits held off the transom but attached to the rail, a 100 lb dinghy with 50 lbs of fuel in it, 50 + lbs of solar panel et/al and all supported by the same pushpit. To brace it I added two cheap Pulpit Anchors (West Marine part # 114785 - $12.99 ea) attached to the outboard vertical bars of the pushpit, high up near the top horizontal rail and ran cables attached on one end to a cheap turnbuckle and the other to a thimble stretched down on an angle to padeyes through-bolted along the top of the coaming, then just tightened the turnbuckles until they had the pushpit held rigid. Voila. Then you can hang virtually any heavy item off your rail.
I'd throw away that old Johnson though and get a lighter 2 stroke outboard, save your back along with your railing.