Usually an instrument fitting in a throug-hull consists of a PLUG which is inserted into the through-hull and then secured down by a locking collar. You unscrew the locking collar and remove the whole instrument "head", and replace it with a dummy plug, when you want to pull the head and seal the hole.
If what you have doesn't have locking collars and doesn't come out...You may have a bozo job.
The wires coming out of those heads usually have had a sealant (could be wax but usually is a permanent sealant) poured in to keep bilge water out of them. They are not designed to be repaired, but if you can chew out the sealant (X-Acto knife or Dremel too) you can probably solder or crimp new wire to it. But I'd guess if the wires were cut, the instruments didn't work, and usually new instruments will come with (and often need) new senders. So you may do a lot of chewing out sealant then wiring and testing and finding out the old senders don't work anyway.
If it IS wax, you can soak them in a pot of scalding water and the water will usually melt and lift off. That will let water in...but if you are completely disassembling them, that won't matter, they can be dried out by pouring in a little alcohol, blowing them out, and letting the air dry.