Solar might work for you but a battery maintainer is pretty cheep and costs little to run and has a better chance to bring up a low battery. About $20 at Home Depot , of course only works if you store boat near a plug
I know that Bombadier is now owner of the old OMC trade names (johnson and Evenrude) and they make the new engines but do they or anybody else supporting parts for the older OMC's?