Welcome to the asylum!
A 24' Excella....can't be too many of them around....
You have the venerable P30 chassis. Parts are cheap and plentiful, but they can be a pain in the a$$ to work on.
Blown motor hey; I hope you got a great deal on the coach because to re&re a motor in these coaches is a royal pain in the pitoot and takes a lot of time. So here goes.......
Question number 1. Is the motor you have rebuildable or does it have a hole in the block?If the engine is rebuildable then doing a core return will save you some money.
All 454's are not created equal and you'll be looking for one with a RV cam and built for an RV. A rebuilt 454 with installation and you're talking $10,000+. I would also think about possibly going with the 4L80e transmission.
Yes you can get a crate 502 with or without fuel injection. These motors are great and RV ready, with big power, decent mileage (especially with fuel injection) and good reliability. However, you are looking at $15,000 for carb and a thousand or so more for fuel injection. A 4L80e would be a slam dunk. Cheifair replaced his 454 with a 502 carb crate engine.
The next possibility is the Chevy 8.1 with an Allison transmission. This powertrain was used in early 2000+ Chevy 3/4 ton pick ups. The below link shows the install. Again you're talking $15,000.
Last but not least is the Cummins swap or Isuzu 6BD1A or T....now these swaps are not for the faint of heart or people with shallow pockets as these swaps take a lot of time and money but are incredible if done right and work well. Back in 1983 the turbo diesel option in my 310 cost the buyer $15,000....IN 1983 DOLLARS!!!