Hello Willw
Those Redstones are made of unobtanium. The ones imported to U.S and Canada were the last of a production line. Some of the last few were assembled by "volunteers" at the factory.
I has the pleasure of disassembling one, cleaning out the copious quantities of casting sand (Supposedly the purview of Indian Listeroids) then putting it back together and running it. There were some problems, probably because of some being assembled with leftover parts from an earlier production run. Seemed like it was a very stoutly designed and built machine, but the last few were not all that great. Now they are no more. The original importers, Utterpower in the U.S. and a fellow named John Ferguson in Canada, have gone on on to other fields . I do not know the original manufacturer.
You might search these LEF threads for the few members that had a Redstone. Maybe they would sell. I doubt parts are available though.
Best bet for a horizontal is a Changfa of the appropriate power.Quite similar engines.
Cheers,
Hugh