John, Onans are awesome engines and the B48G was used in a lot of applications like the JD 420. This place has about any part you will need for it.
http://onanparts.com/
There is also a guy on MTF that sells used parts and this is also a good read on rebuilding.
http://www.mytractorforum.com/showthread.php?t=91204
I also found a guy on Ebay the other day that sells aftermarket parts for them but I don't know anything about him or his parts.
http://www.ebay.com/itm/360848369158...9#ht_82wt_1362
The Onans are a little different and used sleeve type bearings throughout that actually held up well for an engine being 35+ years old if the oil was maintained. The rod design was changed and a new stronger rod is now available and also some engines had a problems dropping a valve seat. Good luck on yours.
I would take a good running Onan B48G over a Mag 18 anyday.