The good news is that the engines are relatively simple and the parts available. However, if you have an engine that you can't start and has an unknown history then assume a complete rebuild - it would be false economy otherwise. If your very lucky you could get away with new seals, rings, bearings and belts, but given the age of these engines then a rebore is more than likely. If you're reboring it then you might as well regrind the crank, put a better cam in it, etc etc. If you are mechanically minded then stripping and putting it all back together is not that difficult.
www.automotive-services.co.uk/
is a company in Northampton whose work I have always found to be of the highest quality and very knowledgeable - but they don't strip or rebuild.
If you are going to go for a start up to assess the engine then a good buy a Haynes manual. This would give you the knowledge to replace the various belts, pulleys, seals, plugs, points etc that you should replace as a minimum before starting and engine thats been laid up for a long time