The .25 uses a replaceable prop shaft that is reversable -- you can screw either end into the crankshaft, depending on the thickness of the prop hub. The shaft is a standard M6x1.0 so you can simply replace it with a stainless or alloy socket head bolt or find a piece of M6 allthread if you want to use the original propnut (locate a M6x1.0 fully threaded bolt and cut off the length you need). The engine probably has taken a pretty good whack to break the shaft, so you might want to check runout.

If you want to stay original, get a propshaft from [link=http://www.norvel.com]NORVEL[/link] or [link=http://www.sigmfg.com]SIG[/link]