You should check the tappet clearances Mike, it only takes a couple of minutes and may save you a lot of messing about if that is the problem. You might also try starting it with the muffler removed, in case it is blocked. Once those simple things are checked we can work in an orderly way through more subtle problems.
Once we know that air is moving freely through the intake and exhaust systems, the ignition system is OK, the valve timing is right, and it has compression, we need to know if it will start with fuel in the cylinder. If it will, we have a fuel system problem. If it won't, we are into relatively serious headscratching and can all have some fun with this.
