Wow, Boregy, you've got a proper little find there. I agree with your initial assessment - it's done very little work. I would almost think a quick lube of moving parts and an engine oil change would be all you need to get cutting again! That's in amazing original condition and under the chain cover is brilliant.
The only things you can't really replace on these today are the rear rollers, bed knife carrier (sole plate) and in your case, the clutch. They are all made of cast iron, so don't go dropping them or running roughshod over bumpy, solid surfaces. (You can get replacement clutches, but they're alloy.)
