I've just had another look at the documents.
The following excerpt is of concern as it looks as though the Historic Vehicle Tax Exemption may be amended to take account of substantial changes, including increases in power ratio greater than 15%.
It looks as though they may make these vehicles subject to Tax, as they will no longer be deemed to be of historical interest!
Quote
"The process for vehicle keepers declaring an old vehicle is exempt from testing.
"Every year vehicle keepers are required to licence their vehicle. Part of this process
includes paying Vehicle Excise Duty (Vehicle Tax). Where the vehicle that year is
over 40 years old at the beginning of January then from April of that year the vehicle
keeper is exempt from paying this tax. The vehicle keeper must apply for the
exemption from tax at a Post Office that deals with vehicle tax. Further advice can be
found via the following link:
https://www.gov.uk/historic-vehicles/vehicles-exempt-from-vehicle-tax
It is intended that alterations be made to these processes to take account of the
“substantial change” rule related to vehicle testing."