I’ve run ringed Diesels too. They don’t hand start for beans, and seemed finnickier to adjust. The ringless engines seem 10X better and easier to handle, so I dumped the ringed ones or converted them to ABC. I recently converted a ringed ST .51 to ABC with a piston/liner from a Tiger Shark .52. I haven’t run it as a Diesel yet, but I intend to. My benchmark 7.5cc prop is an APC 13x6, which an ST GS.45 ABC can do around 10,800+. I’d think a 13.5x6 would be a little small for a .61?