I may be giving false hopes, but supposedly the #730162 flange repair adapter Tecumseh sold works. I thought the port and maybe even the spacing was not the same as the rear exhaust on some of the sidepoppers but looking in my book its showing/calling for the same flange either engine and the only repair flange available for the H25-35. I dont have any side exhausts engines here to double check with the repair flanges I have here. Does not look like there is much meat to make a seal anymore on your block even if it does work though.
They are really thick flanges with NPT that bolt on, I used one for a pattern for some weld on flanges that weren't as thick when I was making H25-35 mufflers up, and have drilled them out as well and used them for some larger heavier headers too.
The one with the paint on it is the Tecumseh repair flange, if the kit is complete when you find one they usually come with the screws and gasket.