I was going to suggest talk to Polar Engineering, but they have closed
and Bowman have taken on their product range.
http://www.ejbowman.co.uk/news/News_polar.htm

From my research when I was looking for a marine engine, the only
difference between continuous and intermittent rating is the power
allowed and duration. I bought a Daewoo L136 from Watermota UK - 160Hp
at 2200 rpm continuous rating. Excellent engine, very reliable, frugal
and well priced and this particular engine is used when re-engining
trawlers whose Gardners are beyond economic rebuild or repair. Daewoo
do a 70 Hp engine - http://www.watermota.co.uk/daewool034.html

Give Mike Beacham a call - you might be surprised!!

Colin Stone