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Wheelsmfg and Problem Solvers both sell hangers that work with track ends. Never used them myself but I'd probably consider this route first before chopping the frame up.
https://wheelsmfg.com/derailleur-hanger-128.html
https://www.bikeparts.com/BPC376453/problem-solvers-chain-tensioner-with-derailleur-hanger -
You'd be better off getting whatever Genesis's geared equivalent is. If you don't want a single speed any more it'll be cheaper in total to sell your SS frame and buy geared. By modifying it you're turning it from a rare-ish frame into just another steel hardtail, but modified so some buyers won't want it.
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I’ve used the DMR version of what @Colonel_of_Truth suggests. Worked fine for me but I was using friction shifters and like 8 (I think) speed components. Modern stuff might be more sensitive.
Dear wise frame builders. I have a Genesis Fortitude Adventure SS with Horizontal dropouts.
I'm considering getting the dropouts swapped to run it geared. Am I not considering anything here? How simple a job is this for someone who knows that they're doing?