What do you want Andy? The GX120 & GX160 were both copied by the Chinese & I've found various bits swap directly - heads, valves, gaskets.
The GX160 is used for go-cart racing & the second hand unused heads sell for pence on ePay.
It has long been Honda policy to find one dealer who is prepared to take on doing spares for this or that model of bike when they get old. A master computer record is held & dealers are asked to sell back their old stock to Honda who will then set up one dealer in each country to deal with (say) twenty year old stuff by post.
Well, they used to do that & they are a very traditional company at heart. You could do worse than ring Honda UK & ask them who stocks their old stationary engine stuff ;o))
The plan was to spend a day or so with the offspring getting him oily with both of these and having a working gen set (scrounged some years ago) and a working water pump (fiver, last farm auction) at the end of it. Neither has started in living memory, I've no idea what's essential or whether they were either repairable, knackered, or only repairable by buying something particular. But a set of new gaskets all round, new plug and a tub of grinding paste ought to have given a fighting chance that I'd have enough on hand already to get at least one running by teatime. Maybe megger the coil beforehand / look for a spark, as a GX120 coil is cheap & easy enough to swap, but not so cheap you'd buy an unnecessary one.
Plan went sprong a little when I realised that the genset (dug from under its vast mountain of accumulated tat) wasn't a GX120 same as the pump, but a G42 (should have remembered the loose rope starter - wonder if I could swap a GX120 onto it?). Now it's two different engines, one with awkward spares, so the efficiencies of scale don't work out. I'll not be taking the head off that unless it becomes necessary.
The tip about old spares is handy, thanks. I'll enquire with Honda UK.
The valves are retained in their spring collars by a curious "figure of eight" shaped hole. They are not strong & fingers alone are needed to fit them - I suspect that valve float will stop the little beasts from destroying themselves should the governor give way.
Me, I'd just fit a brand new plug, clean the carb out properly new petrol & I bet it goes. Slosh some petrol round the drained sump, fill with modern oil & it'll be as fit to run as it was 20 years ago.
I recently purchased a couple of twenty year old gennys from Dudley Simons. Both go, both generate although the 1kva is a heavy lump compared with the modern two stroke 800 watt Chinese job I use for PA work ATM.
Honda's are pretty reliable things. I guess the first thing to check is do they turn over easy, and is there any compression.
I bought an ancient (1980's vintage IIRC) Honda genset a while back. It would run but didn't have any grunt, compression was almost none existant.
The electrics (it had remote start and an autmatic choke) had been got at, found diagrams on the web. Sorted the wiring and I thought the auto choke unit had failed as it wouldn't restart even after cooling (not that it had got particulary warm) for several hours. Next day, properly cold, it would start, it just wouldn't restart when warm...
Looked in the Yellow Pages for a local Honda spares place, rang 'em up and found I could still get the auto choke part and a set of rings and gaskets. Decided, in the end, that the effort required wasn't really worth it and this set was destined to be a backup "it'll just work" set so took it back.
So parts for ancient Honda engines are available and they don't cost an arm and a leg. Don't forget that Hylomar (mer?) Blue will replace most gaskets or a bit a cerial packet. Probably not for the head gasket on an engine you want to use though.