This must not just be a vintage airplane problem then. A lot of newer classic airplanes, like the C152 and C172 from the 70's likely have updated avionics. Anything that goes on a vintage certified airplane in the USA
Yes you're right, I'm not saying it's just a problem with the 140. My 172 would have trouble joining the G-Reg, just on it's battery alone (although this would be pretty cheap to fix). It's just the older the aircraft the more likely mods have been done to bring it more up to date, and as things wear out improvements have been made instead of just replacing like with like.
Our C140 has an intercom, a newer radio & Mode C transponder, shoulder harnesses, wheel extenders (which is a Cessna thing, not aftermarket), auto fuel STC (which is paperwork and stickers, nothing installed besides the stickers), and a metalized wing. All the aftermarket stuff (metalized wings, shoulder harnesses, auto fuel) has an STC associated with it, all the new radios are TSO'ed parts. It's all in the logs signed off by an A&P. This is typical in the states.
You just have to make sure that the paperwork is all complete and in order. ANY mod will be checked and if not 100% correct it will have to be removed, replaced or refitted. I believe the first you will know of problems is when you have de-registered from the N-Reg and BEFORE you get on the G-Reg, as this is when the CAA (or EASA?) come to inspect the aircraft.
Going back to my battery, it is a Concorde sealed battery. This has an STC & FAA-PMA approval for use on the 172, but they are still not approved (or were not two years ago) for use on G-Reg aircraft certified aircraft. This may have changed, does anyone know if this is still the case? The same was for HID lighting, approved for FAA but not for CAA.