The machine would've been built ready for a change in the law allowing for higher jackpots. What's the date on the glass?
Some machines did change the entire win plan structure, especially if it has been built for the 'under 18's' £5jp too. This therefore needed a set of interchangeable decals. Not all games did this though, opting to change just smaller number of them - like the jackpot and the top feature. Instead they could 're-glass' them with updated ROMS for the lower stake prize market (we've spoken about this before, with JPM and Crystal, I'm sure!).
This pic shows the changes in Law over time:
View attachment 283827
In my opinion, changes that made jackpots higher were the nail in the coffin for these types of machines. They made the stake

rize ratio progressively worse over time. By that I mean 20p play @ £15jp was roughly 75:1 ratio. The same machine with a £10jp would've been a 50:1 ratio. Early 90's games we 20p £4.80jp, so
24:1. A higher ratio made a big difference to the game play. Long stretches of stale and uneventful gameplay. Lower ratio and you can easily pump in a jackpot's worth in 24spins, so it should be ready to pay at least something back!
There's a guy on YT who makes good content about Fruit Machine emulation, old emptiers and just general history, from his perspective living in the NorthWest in the 90's and now the Isle of Man.
There's one about the changes to Jackpots over time (where I nicked this screenshot from):
And another about Temple of Treasure!
Also, MPU5 is regarded as being "bulletproof" tech, so I'm surprised it's not working to be honest. You can get replacement parts fairly cheaply and easily too.
Good luck!