If your engine runs fine I wouldn't replace it. Have you done a compression check? Where is it leaking oil at? Can you post any pics of it?
Good source for Onan parts:
Boomer (The used Onan parts Guy)
He is on MTF & WFM. Do a search on his name and send him a PM about your issue. This guys knows a ton of stuff on Onan engines and is very nice to talk to + he has a ton of reasonably priced parts.
https://www.mytractorforum.com/searc...rchid=30508943
https://www.wfmachines.com/forums/forum.php
Gary's Place
Hope, MI 48628
(989)689-5092
https://www.onanparts.com/
I did a complete rebuild on a P220 for one of my 982's here. These are a little different than a B48G but pretty close. Most of the parts were purchased at Gary's Place.
https://www.onlycubcadets.net/forum/...ad.php?t=42394