I once met another boater who had a similar situation where the original pan was leaking pretty badly, but the unit was functioning well otherwise. A replacement pan was no longer available so he removed the unit and cleaned the pan as well as he could. He drilled several more drain holes on all four sides. Then he treated it with some kind of rust inhibitor like Ospho and coated with Rustoleum I think. Anyway, he then placed a slightly larger polyethylene utility tray under the whole thing and re-plumbed the drain line to the new tray. I'm not sure how he secured it all, but he must have used some rubber washers or seals of some sort when he bolted it back into the boat. He said it had lasted a couple of years so far. Seems like a lot of work, and it would be a band aid for sure, but it might be an option for a while.
FWIW, my vote would also be to replace the original pan if it is still available from the manufacturer.