the sad central fact of this saga is simple--the ins company does NOT value the car the same way its owner does. to them it is just XXX year XX Brand @ xxx,xxx mileage, therefore the value is $xxxx. period.
Like it or not, they could care less about what you say car is worth. there is no statistically valid way to value the mods, so the ins co simply does NOT value them.
this is the downside of making a large investment of time, emotion, and especially $$ in modding your car. if/when it gets terminally crunched, the owner will NEVER get that full amount back, no matter how well the mods are documented.
buy back the wreck and part it out, that is your best way out of a tough spot financially. do NOT try to rebuild the wreck, there will inevitably be more repairs than initially thought, and owner does not have the time and resources to deal with that crapola.
So suck it up, deal with the cards you been dealt, and move along. this decision will save you a lot of emotional wear and tear. (not to mention it will get you out of the effing rental sooner
)
condolences, and I mean it.
PS--at least if you get another bugeye, much of the good stuff will transfer over no prob.