If you invalidate your insurance by not notifying them of a mod, that means you're no better than pikey sc*m who drive dodgy cars around without MOT or insurance. And it means if you have an accident, the other person won't get a payout from your insurance company.
How would you like it if another driver hit you, you expected a payout to fix/replace your car, but their insurance wouldn't payout cos they'd modded the car but not told them?
You'd be fuming.
If you can't afford the insurance, you can't afford the mod.
Man up, pay up and pay the full cost of the mod.