If you replace the original engine with an engine that is broadly similar to the one originally fitted or offered as an alternative at the time then I suggest that you are well within the spirit of the regulations.
I agree.
I've got a 72 Hillman Avenger GT (don't laugh) which has had a 1600 CC engine fitted in place of the original 1500 CC unit. The engines look identical, the only difference being the bore and stroke.
I did this swap in 89, 2 years before I took it off the road to restore it, which I am still planning on getting round to doing at some point.
I will be getting it MOTd though, at least when it first goes back on the road.