In researching options for scratch builds I have found the original manufaturer of the XB-600 controller (ShenZhen Sucteam). The controllers they offer are based off of the Infineon microcontroller. They currently offer 48V/800W, 60V/1200W, and 75V/1500W controllers with customer selected options such as regen braking, regen braking controller by throttle, low voltage cutoff, and max current limit.
So if you're too squeemish about having to replace all of the mosfets and capacitors in your 60V/800W controller in order to reach 72V, you can simply drop-in one of their replacement controllers which come with 100V capacitors and 75nf75 or irfb4310 mosfets.
Ebay store: http://stores.ebay.com/HuaQiang-North-Road_W0QQssPageNameZstrkQ3amefsQ3amesstQQtZkm
E-mail: e-crazyman [at] gmail.com
New Website: http://www.urebike.com/
He's really helpful, but be aware he only speak Chinese. He manages fairly well using translations.