INSCO's should be prohibited from voiding claims because "Modifications" unless the alleged "Modification" was a significant factor in the events leading up to the damage.
Actually, Compulsory insurance should be scrapped altogether. Motor insurance is pretty much the only private sector product that one is legally compelled to buy if you want to live a "Normal" life.
RTA aspects should be provided for by some other means (Historically schemes like this used fuel levies. But with the uptake of EV's and hybrids some other method will be necessary, perhaps an annual driving license fee?)
Insurance of the vehicle would then be optional and up to individual drivers. (Since almost all accidents are pretty much 50/50, people can chose to claim against thir own insurance for damage sustained or not as the case may be. A fund/appeals process should be available for those extremely rare cases where somebody is indeed genuinely blameless)