So what have you done in this respect, here are some of my more extreme fixes.
So far
Lima 50 chopped up bogies and fitted white metal frames, bogie is 50% original, 20% brass sheet & tubing, 20% filler (I think it was Plastic Padding) 5% Araldite 5% Lima gears from spares in a shop.
The other 50 needs similar attention - I will be making a new chassis one day.
Hornby Flying Scotsman, correct height, wire handrails, daylight under boiler, 5 pole motor. Horby 47 bought as a bodged wreck - drilled out bogies, sides smoothed, flush glazing, now 47513 Severn, looks fine next to my Heljan one.
Lima BR1 - mainly fitted with correct size wheels - ride height changed and smaller coupling fitted Lima B4 - couplings fitted to other end of bogie and recessed for closer coupling. Lima CCT - proper diameter wheels and converted to compensation Triang Hornby Thompson - Scratch chassis and bogies to look like a Bachman model, original Mk1 chassis shortened to 57' and fitted with etched BG sides and an ally roof.
Airfix 2D TSO converted to 2E by moving toilets, screwed one up doing it the wrong way - that had three swaps done!!!! Half of my fleet are 2E, so much better models than Lima 2Fs.
(I have enough aircons for two sets)
Will not mention converting Airfix minerals into MCV and MXV
As I replace my Lima BGs I am turning them into restaurants. My Bachman one got fitted with B4s to go with Mk2 stock.
E1883 is currently in the works!!