Interesting reading, for me at least
Here is paragraph from the "Acceptable changes" section
"They are changes of a type which can be demonstrated to have been made when vehicles of the type were in production or within 10 years of the end of production"
So, if I read this correctly, as the 4/4 was in production until this year, one could fit any engine that was used during that production period or might have been used during that period.
It can be demonstrated that the Vauxhall engine conversions were done during that period so could be used now and still retain historic status.
I guess rules can have many different interpretations to suit the user.
