Please help me identify my first engine

I hope someone may be able to help me, I am new to stationary engines - this being my first acquisition - and I would like some help identifiying what it is, and what is missing.

I have been on the lookout for a small stationary engine for a little while, and found this lurking with the lawnmowers at my local council dump today. I secured its release for the pricely sum of £1

I hope someone might be able to identify what it is, and what parts are missing - based on my knowledge of lawnmowers it lacks a fuel tank, carburettor and recoil starter at the very least. It appears to be a four-stroke engine, the oil is clean and the main body of it seems intact. its clearly sat around a while as there was a lot of oxidation on the ally bits, most of which came off with a stiff steel brush.

it seems to turn over fine, and has compression as I can feel the pressure release in the exhaust stroke if I hold my hand over the exhaust and turn the engine by hand.

All the markings it has are in the pictures below. From Googling '86-13 AM' it might be a JAP Model 5 as this looks similar to what I have in front of me. The other suggestion is that it might be a Villiers C12?

Are these a good engine for a beginner to repair/rebuild, and what is parts availability like? Here are some pictures of what is in front of me

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thanks in advance

I'd guess at a JAP.
The AM may indicate it was once owned by the Air Ministry.
Nice little engine to start on - you'll may find that the magneto coil needs replacing. Carburetter you can probably pick on on e-bay. Try L&S Engineers for other bits. Best of luck.