First off, check the wires, particularly at the plug before you give up on the existing one. They tend to pull out, Tamiya specifically recommends using needle nose to remove that plug. I had a Tiger flash stop working. I tested the unit in another tank and it worked fine. While searching for answers I found it was a common problem with the MFUs after time. I never got around to sending it in for service and after swapping abunch of stuff around, I looked at it again one day and found the plug wasn't sitting right in the MFU and it's been working ever since.

Double check, then if possible, try the flash unit in another tank to determine whether it's the problem or the MFU.

HL makes a flash unit, but they're hard to find in stock and I wouldn't begin to know how to wire it to Tamiya.