Yes, just as you have to let your insurers know about modifications you make to a modern vehicle, the same goes for classic cars.
In fact, many classic car owners make more practical upgrades than most to their cars - things like suspension, brake and engine upgrades, power-assisted steering, electronic fuel injection and uprated radiators for improved cooling.
Not all standard insurers offer cover for classic cars, but there are plenty of specialists that do, and they’re used to covering all sorts of modifications.