I don't think regs for the TT have been slightly close to regs for the Manx for quite a few years. Certainly not since the production based bikes became the mainstay.
At the TT you can run slicks, you can't at the Manx.
The scrutineering requirements are different at the TT.
Now days the class regs for the TT as the same as British or world championship regs, only exception being the senior as it has no comparable class anywhere else, that’s how the Paton got in last year. For the Manx there is just a capacity limit.

The numbers thing has a whole load of different elements.
Firstly last year or possibly the year before the FIM/ACU changed the regs on number to allow them to be mounted on the side of the fairing as well as the seat.
The TT runners using there own numbers is quite a new thing. In my opinion this a cascade effect from MotoGp etc running whatever numbers they want as the timekeepers don't need to see the numbers due to the introduction of transponders. This however doesn't help the spectators and more importantly the marshals.
It wasn't that many years ago that one of the front runners was allegedly turned away at scrutineering due to having a non supplied number so he went and got the supplied number pulled the whole backing off slapped it on the front of the bike roughly smoothed it out and off he went.
I wouldn’t worry about shaping or cutting the numbers down. If you're not doing it you're probably the only person who isn’t.