Starting Problem Pioneer XF200-2V

Hello.

I Have been having starting problems with my 2006 Pioneer XF200.

The first problem, about 2 years ago, was caused by rain water getting into the clutch lever switch, which I by-passed and it cured the problem.

Now I have a similar problem where the starter motor either jams or spins freely.

I can get a compatible replacement starter motor for about 45 GBP (Suzuki GN125 type) but I think the problem lies elsewhere.

The engine has run very well since I bought it but I have never adjusted the valve tappets, partly because there are no timing marks visible through the adjustment cover and I do not wish to adjust them by guesswork.
Could the valves have gone out of adjustment so as to cause such a problem?