They're not really worth much, and early ones with points ignition can run as low as 16-1 two stroke mix....not good for the canal at all, there will be a trail of oil and dead fish behind you.
pretty much 1930s technology. and designed for heavy work boats, so low gearing. high thrust but low speed.
it would be OK for free, i wouldn't pay more than a few quid for one, unless its a really desirable one.
there is plenty about them if you google it, SOS website is good (saving old seagulls)