I've looked again at the engine, and short of removing heat shields, the alternator, etc, I can find no model number stamped or fixed to any part of the block or elsewhere. All I can see are the words "Honda 13.0 OHV" on a label on the top of the engine.
However, I have been online and the only13hp engine I can see produced by Honda is the GX390 (horizontal) or GXV390 (vertical). Apart from the way they are modified to be mounted I assume they are the same 4-stroke single cylinder engine, and on one of the websites I visited theyare stated to be commonly fitted to ride-on mowers, etc.
So on the assumption that it is this engine, what should be the valve settings, please, where on the engine is the valve cover, and what needs to be removed from the engine to get at it?
These should be my last questions, and as I've already said, I'll then give you an excellent rating. Thanks for your advice and patience.