dont file the prop down mate it will seriously weaken it and make it dangerous.far better to get a new crank or better still another engine there are lots of engines out there for good money especially on ebay.i got an sc 61 for £25 on ebay pristine condition too.ive bought engines for a tenner on ebay before now and all of them have been good engines.some came with a prop too.
heres a cpl available now last one has wooden prop a big no no on an air boat!!.
[link]http://cgi.ebay.co.uk/JBA-61-Two-Stroke-Aircraft-Engine-NEW-99p-Start-NR_W0QQitemZ300063687610QQihZ020QQcategoryZ34055QQ rdZ1QQcmdZViewItem[/link]


[link]http://cgi.ebay.co.uk/OS40LA-BLUE-MODEL-GLOW-ENGINE_W0QQitemZ220065728844QQihZ012QQcategoryZ340 55QQrdZ1QQcmdZViewItem[/link]

[link]http://cgi.ebay.co.uk/SUPER-TIGRE-S-40-R-C-MODEL-AERO-ENGINE_W0QQitemZ160067605456QQihZ006QQcategoryZ340 55QQrdZ1QQcmdZViewItem[/link]