Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Yes, an Onan can replace it.
We can not give you an estimate on cost as there are to many variables involved.
The cost depends on exactly which model Onan you would go with, what options you might get for it, and how much modification would need to be done to install it.
This one wold have to be physically seen by someone to give you an estimate.