What mods are "accepted" depends on whether you are a buyer / seller or a keeper.
All of my Lotus have mods to make them more fit for my use. As I am a keeper who keeps and enjoys my Lotus and not a seller, I dont really care about the impact either positive or negative these mods may have on the market value just how they affect my enjoyment in using the car.
In the long run from a buyers / sellers market perspective an orginal car will always get more in the same condition as a modified car unless the buyer is a user who want to use it in the same way as the seller and would do the same mods as already done. As most users these days of Plus 2 are like me and are about weekend "fun" use then most of my mods would attract a good buying group if I ever sold (which i will not be doing).
Key "mods" for a plus 2 in weekend fun use for me are:
1. CV drive shafts (reliability and elimination of surge)
2. Electronic ignition ( reliability and ease of maintenance)
3. Electric fuel pump ( ease of starting after periods of sitting)
4. Modern sticky tyres ( fun in the corners)
5. Spyder spaceframe subframe (cant call it a chassis - it not really required but when the old chassis expired due to rust in the backbone i just liked the spyder design better, its a little stiffer and maintenance is easier with it also)
6. Uprated engine using ported head, bigger valves and hi lift cams to around 150 hp ( building nice twin cams is fun)
7. Better sump baffling (to avoid oil surge in righthand corners with those sticky tyres).
8. Preengage starter ( reliability and ease of starting)
9. MGB centre muffler in exhaust system ( keeps the noise to levels that dont attract to much attention )
10. 3 row core in standard small radiator with modern ducted electric fan ( keeps it cool on hot 40c plus Australian days in heavy traffic)
As mine is a late Plus 2 three things I have not needed to do is fit a five speed as it came with one orginally, if it did not have one, I would fit one from the options available, orginal Lucas alternator so no need to replace generator and I have not modified the suspension which is still superb for road use even with modern sticky tyres.
I also dont call new paint, new air in the tyres, new modern oils in engine and gear box etc mods!!!!
cheers
Rohan