The 1912-14 Excelsior Autocycle model 4 engine seems to be the most plentiful, but you're still going to have to shell out at least 2 grand if you find one.
And most likely, it'll be missing the magneto and the carburetor.
Each one of those items will run another thousand or so.
Like Shawn said, at that point, you're building yourself an antique motorcycle, so don't stop there.
A complete 1913 Excelsior Autocycle isn't too much further down the road.
Another 10 or 20 grand, and you'll be rolling.