Have a look in Halfords. I think this is the 2.3 litre four cylinder petrol engine. If so, it is basically the same engine as went in the 240 series Volvo cars (tanks!). You will also find that Halfords are a good place to source plugs, points and oil filters. The major difference is in the cooling system where you have a heat exchanger arrangement as opposed to the radiator found in a car. Otherwise there is little difference.
The engine is a 2.1 litre B21, the same engine is (as nick says) fitted to the Volvo 240 car.
You can get a manual specifically for the Penta engine from Keyparts in Watford (£20 ish). I've just replaced a dead engine with one from a car and they're almost (but not quite) the same. If you need any more info get in touch.
I have found an old book in my boat with the same engine titled
'Do-it-yourself-Aquamatic' by Volvo Penta, Publication nr:7730252-9.....10-1985
All the pages are stuck together so it's not much good.
It came from John Hawkins Marine at Rochester Tel:01634 840812 if they are still there.
Used to have an AQ140A four cylinder 2.3 in a birchwood 22. Twin carbs etc. Have a photcopied manual purchased from a service agent that you can have if you want it. If you give me your address I'll pop it in the post.
If indeed you have the AQ145 which was based around a 2310cc unit (B.23) then either the AQ145A or AQ145B require a min fuel octane of 95 (A) or 90 (B) as such unleaded or leaded (LRP) is perfectly correct to use.
If you want one new copy of the DIY Aquamatic manual let us know.
A or B? If you still have the decel stickers on top of the black plastic air box, it will say there, if thats missing does the engine still have its ID plate (gold & black plate) on the side of the block there you will find a serial number from that I can tell you A or B.
The manual, how much? Put a donation into your local R.N.L.I collection box and I will put a copy in the post, email your details to: email@example.com