Missed this thread originally. Dude, I’ve the same model, a Brit 1991 ABS. Here’s a thing. I heard that the MOT people can give you a really tough time if you remove the ABS and insurance companies take a dim view of brake mods especially (even though we FJ owners know the superior blue dot/gold dot set up). Have you encounter any such grief from the authorities?
Yes, insurance-wise, if you don't declare that you've removed the ABS, then your insurance would be invalidated - if the insurance company found out. If you do declare it, some wouldn't insure you and others would hike your premium.
A switched-on MOT tester here would definitely fail the bike for not having a working ABS system, but if the front wheel &brakes had been changed and there was no evidence of any original system, the I wouldn't be an issue.