I think you definitely could test the bike with 3 12-volt SLAs. Just make sure the rear rack can handle the weight before you mount them and be careful about hitting bumps while doing a test ride with 3 SLAs on board.

I have seen posts on other forums by people who run Elite Power's LIFEPO4 batteries without a BMS , but you have to be especially careful while charging. Your battery may not have an optimum life span, but then you bought this as a non-runner so, hopefully, you're not out-of-pocket too much. It might be preferable, if you're comfortable with electronics, to disassemble the battery pack, examine it to see if you have bad cells or soldering and, if necessary, beef up the BMS using a compatible one rated to handle the maximum continuous discharge rate of your controller (BMS boards have improved, Ping batteries out of China are now using version 2.5 of their BMS). Good luck.