A cars worth whatever someone's willing to pay and with the ISF being a rare car it would actually appeal to people who are researching them and like to modify, I know if this was on the market before I bought mine I'd snap it up as before I bought mine I researched everything including what modifications could be done.
But yet you can find highly modified supras skylines infact most Jap stuff with a bit of a following and it does make the car more valuable than a stock example, so why would this be any different at all yes they may take time to sell but they do sell I'm talking from experience several plus times. An example of such car
https://www.ebay.co.uk/itm/Nissan-Skyline-R34-GTR/264056141141?hash=item3d7af8f555:g:bYIAAOSwtKlb-AJi