Update, I tried a 14x13 Turning Point Hustler, put me at 6200 rpm wot, very quick on bottom end but very slow at around 18 mph. I used the Turning Point MasterGuard hub kit, very clunky in and out of gear.
I bought another Hustler 13.75x15 and a Solas Rubex hub kit. Much better, 5500 rpm wot...