If 31 is the length ..... a 7,5cc/.45 engine is allready a hand full to manage.

A good bargain .. all American .... enough parts ...... Mecoa 'House-brand' [link=http://mecoa.com/mecoa/46/4622.htm]7,5cc/.45 pullstart-marine[/link]
@ 130 us$

At the same company K&B 7,5Inboard @ 280us$ ... you'll need a Sullivan starter and batteries.

The top of the bill are the (usa)[link=http://www.cmdracing.com/updated/mac45.htm]MAC[/link] (from CMDI-Andy Brown) and the Italian [link=http://www.cmbdeutschland.de/shop/start.htm?cmb_modellmotoren_marine_und_ersatzteile _cmb_.45_rs_(direkt).htm]CMB[/link].

Both are in the upper price-range.