So off with its head (!) and there's the inlet valve with a lump of muck stopping it closing, and quite a lot of muck all round. Not that surprising considering that the machine (Landmaster 100) is seriously old and has to have its oil topped up after each use, but it goes and until today went well.
Anyway to the point ! It's I believe a Briggs and Stratton engine - the cylinder head came off cleanly and judging by the missing washers may have been off before. The query is do I need to use gasket goo when replacing it - the gasket is in good condition. I guess this is a genuine B & S engine not one of the modern Chinese clones.
I was amused to see that the speed regulator works off a blade in the cooling air flow - crude but obviously adequate.