I’d stay away from the 11hp(25ci) unless you’re able to re-work it and make it flow as much or as good as the 12hp (28ci)head , but once you think you have enough flow, get more, flatheads don’t need the compression that OHV’s do to make power the need the flow to get the fuel mixture into the cylinder, do some searching on here there’s lots of info on flatheads!