I have a fairly bad-ass 93" iron sportster (street) that is at the end of it's HP potential in current form. It is 3.5' X 4-13/16", 12:5 to 1, Red-shift 550 grind, Super 'D' w/ two t-jets, etc. Zippers did most of the engine/trans.
My biggest limitation I think are the cylinder heads. They have the big XR valves, dual plugs, port work, etc. The ports can only be enlarged slightly due to the thin-wall design and do not flow nearly enough for the combination I have.
What I really would like to find is a set of Thunderheads in any condition to go to the next level.
For those who don't know these are basically aluminum copies of the stock iron head with better valve angles, combustion chamber and port size. They were manufactured by a small company in Texas back in the late 60's to 70's
There were quite a few sets sold over the years and I missed out on a set a while back.
So if anyone knows a better solution or the where-abouts of a set of these heads please let me know. I will even pay a substantial finders fee