I have been running a 36 volt SLA with my EZIP for about 200 miles. No problems so far, but I don't cruise any faster than I could with the 24 volt set up (About 18mph). Is your new motor brushless. I saw a 36-48 volt 600 watt BMC that is supposed to bolt up to the EZIP. It is brushless and comes with the controler for something like $350.00, can't remember for sure.
I am interested in your results with your new motor. What is the model? What lifepo4 did you get, and how many ah?