The short answer is that Landrover engines of the period, other conversions "of a type" that were done at the time and engines developed from the engines of the time are "OK". 3 years on and no-one in officialdom has challeneged that interpretation.
Eye on the prize: A vehicle that is low enough to allow your wife easier access. Any other mods that don't have a direct bearing on that outcome are just making life difficult.
^^^This is the critical point.
The magic word in the interpretation is 'type'. I am not an expert here, but it appears that the interpretation of the rules as written allows for replacements to be made using an equivalent available at a point in time. The Land Rover family of engines, through to the TDI, is essentially the current equivalent at any given point in time, ie if your engine blew up at a particular point, the then current model engine would be a reasonable replacement, which then makes that an acceptable change in the future. Also, because of the ease of conversion at certain, early, points in time, the Essex for example met a need and became an accepted option during the working life of vehicles, making it still acceptable.
Fitting a 1.3 Marina engine was never a historic conversion choice, so it is not an accepted type and if you do this now you instantly lose the MOT exempt status.
The bigger issue is with the chassis. There is quite a lot to be learned on this point from discussion around 1500 MG Midgets. As the gearing is generally rather low, there is a tendency to either fit a 5-speed K type Ford box, or an overdrive as originally fitted to the Spitfire which had the same engine. In either case, some adaptation is required to get it to fit in the gearbox tunnel, including cutting a drop-down section to the cross-brace (the Midget is mostly a monocoque with a couple of chassis-esque crossbraces). The general consensus is that technically, if you do this, you have modified the chassis, meaning you lose both the MOT exemption part and the historic status. There are a lot of vehicles floating about where this was done and the odds of being caught are small, but that doesn't mean you are the right side of things if it does get picked up. I doubt you would even pass a single vehicle inspection, at which point the vehicle cannot be made road legal without taking it back to original.
For a Series Land Rover, it appears to be accepted that a replacement chassis is acceptable from a range of options. These include alternative NOS designs which would have been an option, and correspondingly the use of a modern replica of any of these, or a modern, simplified generic chassis which does not necessarily have all the original detailing. What it appears you cannot do is cut bits off of this chassis. Technically, this includes retro-fitting a detachable gearbox crossmember (although the fact that a chassis with such a feature was a practical option at a point in time means you can use a new chassis which has one fitted) or cutting off the old engine mounts. What it appears you can do is add additional engine mounts or other additional features - welding additional bits on appears to be acceptable in general. What you also cannot do is design a new, custom chassis which is not of original type, although where precisely the line would be drawn on that is debatable - for example, adding drain holes for galvanising or to inject waxoyl into the chassis and allow water to drain out appears to fall one side, whereas moving outriggers around or specifying different engine mounts appears to fall the other side.
British law works on precedent - the way the rules would be tested is that the enforcing body would interpret one way, the person who has done something to a different interpretation would fail to comply, there would be a court case and the judge would listen to the evidence and make a ruling which would then stand for future reference. It is likely that nobody has tested the finer points of some of what constitutes 'original' for a chassis yet. Personally, I would not want to be the one to test it as I have better things to do, would not want to wait as long as it would take to get through the court case (could take years with the current backlog generated by COVID) and would not want to spend that much money just to find out. However, each to their own.
Alec
