Go with a MTB fixed/free hub that has 130/135mm spacing. On-One manufacture a cheap one that should be good, as do Goldtec (not cheap)... do some Googling.
Do you want it fixed/fixed or fixed/free... you mention both in your post. You should be able to find both and remember that the fixed side/s could be bolt-on and not necessarily threaded?
Re-build the wheel around the new hub or build an entirely new rear wheel, up to you.
Go with a MTB fixed/free hub that has 130/135mm spacing. On-One manufacture a cheap one that should be good, as do Goldtec (not cheap)... do some Googling.
Do you want it fixed/fixed or fixed/free... you mention both in your post. You should be able to find both and remember that the fixed side/s could be bolt-on and not necessarily threaded?
Re-build the wheel around the new hub or build an entirely new rear wheel, up to you.