Welcome aboard. Glad to have you. Sorry to hear about the troubles. That is rough.
A quick education that I usually give newbies. If Clifford (aka luvrbus) says something, you can go to the bank with it. He has forgotten way more about just about everything that involves buses, then I will probably ever know. So if he says something sounds fishy about your engine diagnosis, I would take to secs and track that down. Maybe you just explained it wrong. Maybe the mechanic is trying to get more money out of you, by spinning you a plausible sounding story. Maybe nothing at all, but I would slow down and tell the guys on here just exactly what is going on. They can and do help a LOT. And if luvrbus says something, just sit down and listen.
Just my .0002. FWIW YMMV
Thanks for the welcome and I will take those suggestions to heart. Several folk have already been very helpful here. The 'rebuild' starts Friday through the weekend. I'm sitting in on the whole thing ( even if I have to pay extra).
. I'll let ya'll know how it goes.