There is another consultation just started on exempting older cars from MOT. Worth a read, and worth submitting a response if you have a strong opinion.
Be aware one option proposes exempting all vehicles 30 years old, but the government's preferred option is 40 years old. Options are also to test all vehicles, or to have a safety test.
A chassis, front and rear suspension and axles would garner sufficient points to avoid being classed as a radically altered vehicle and thus would classify the car as not being substantially modified. One could even use a new chassis!
It would all be open to honesty, and you can't rely on that these days.
If you rebuilt your car using a new chassis, i.e. an absolute copy of the old one, then who would know? Likewise with the engine. In fact any component.