Hi
The Merco will be a "61". It will probably be a 'glowplug' engine (in that the "spark plug" has a coil of wire rather than an electrode with a gap). A glowplug engine does not need any ignition equipment in the boat, unlike a petrol engine, only a 1.5 - 2.0 volt battery for starting. These engines run on glow-fuel (methanol).
If it has a screw-down crossbar at the top, it is a diesel engine.
Mercos were designed and built in the 60's so are a very old design and only used as "sports" engines due to the low power output. Some information about the company can be found here -
http://www.iroquois.free-online.co.uk/merco/merco.htm#TOPJBA are a more modern engine so should produce more power - however - these engines are Chinese, and although the quality has improved considerably in recent years, the early models were very poorly made and unreliable. Infromation on the company products here -
http://www.jba-engine.com/index.aspIf the boat was already fitted with one of the engines, I suggest you try that first and see if it works and gives you the speed you want. If it doesn't, then try the other or even consider getting a more modern engine from say, Just Engines (the ASP has come on in leaps and bounds but is still cheap).
As for any other bits - your local model shop is unlikely to stock a lot of fast marine products, but depending on where you are living, there are a few good suppliers for most of what you will need (may have to be mail order though). Things like gaskets, you will have to make yourself (brown wrapping paper is a good material).
Hope that helps,
Danny
BMPRS Chairman