You've got some factors working against you on this that I'll explain before offering a possible solution.
1. The 6 hp cast Iron engine original used on your tiller was dropped out of the product line-up years ago due to the high cost of cast iron and the market's desire for higher horsepowers.
2. Tecumseh is out of business, although what was left of then was bought by Certified Parts and they are working to supply parts but no engines are being built at this time so product availability is getting scarce.
3. The HM80 8-horsepower engine was not compliant with emission regulations so production ceased early.
4. The original engine you had probably used an extra PTO run by an extended camshaft to get reverse.
5. The Horse tiller was built like a tank and outlasted just about any engine you could bolt onto it, but Troy-Bilt went bankrupt years ago and what was left of them was purchased by MTD.
With all that said, it you are looking to repower your tiller, you have about a 90% chance of success using the HM80 that Small Engine Warehouse offers, but you might have to do some modification to get it to work. In most cases you have to lose the front bumper if equipped and in some cases you have to modify the mounting plate to accommodate the larger footprint engine. Another option is to use an engine without the auxiliary PTO shaft, but you'd no longer have reverse.
I wish the answer was better, but I can only tell you the truth of the matter.
Small Engine Warehouse
You have a great tiller there if you can get a running engine for it. Better than anything being made today. Best of success!
That gives you quite a few more options.
Check the link below. Small Engine Warehouse has done a great job with supplying repower information.
Of the choices, Kohler is the highest quality and the most money, so there's a tradeoff depending on your budget. The Robin and Honda engines are good choices as well, and Honda parts are easier to find.
I would probably tend to shy away from the Tecumseh, only because the parts are becoming a bit more scarce. Certified Parts bought the engine division but are no longer producing engines even though the plan is potentially to do so. That leaves Tecumseh's future pretty cloudy at this point.
If it were me, I would think a good middle-road choice between quality, expected life, parts availability and price is the Honda GX200QX. It's an overhead valve 6.5 hp with a cast iron sleeve in the cylinder for longer wear. Be sure to disconnect the low oil shutdown if you use this or any other engine so equipped.
Hope this helps!