At the risk of sounding a bit oversimplified here, A LOT.  Be careful as you know that, but not being able to drive it, there may be a great deal more.  Incidentally, a blown engine is a good reason to get rid of a late model bus, as that will usually make or break one's love for said bus, depending on how deep their wallet is.

 

It is fairly easy to convert a late bay window bus over to type 1 power.  The only problem is that a late bay window is a tad bit more weight than a stock type 1 was designed to push around . . . as long as you don't mind a slightly sluggish bus, they are a LOT less expensive than type 4 engines.

Something else to consider is the "Blown" engine that is in it.  What exacly is wrong with it?  It may need to be replaced (kind'a likely) but it might be an easy fix to get it going again (not that likely, but it happens).

People's definition of a "Blown" engine vary wildly!  When I refer to an engine being "blown" it means there have been major moving parts that bent, broke or locked up while the engine was turning at high speed.  There may be a few parts here and there on the engine that might be salvaged, but for the most part, the engine would make a good boat anchor.  However, I aquired a "blown" engine a trade once that was perfectly good . . . turned out the rocker shaft on one side was loose.  I torqued the nuts down, bolted the engine in a car and drove it for years.

. . . if you wanted to do it right I would look into one of Jake Raby's engines. Seriously.

Without a doubt, Jake Raby can build the toughest, most reliable type 4 air cooled engine out there.  The only drawback is that he knows it and charges accordingly.  :shock:  But if you can afford the best, like Russ said, talk to Jake.