In terms of modification, because the basic design of the 140 is so good, a lot of gains are possible with quite modest changes.
This is Olga, my old 144S on the Nurburgring. She's now owned, and driven in the video, by Ryan (user name here Hemihusky).
I made some subtle engine and suspension upgrades, nothing that undermines comfort, usability or the original character of the car. (And all reversible if he ever wants to reinstate slowness and worse handling!).
Have a watch of the in-car video, link below. You'll see that handling is responsive yet benign and stable. Still not a high power car, in modern terms, you can see she struggles to regain speed after being blocked on up-hill sections, but she performs very well for a 50 year old family saloon, still on 165 tyres.
Even getting blocked a few times, this is an 11:18 lap, thanks to small mods and of course Ryan's good driving!
https://m.youtube.com/watch?feature=...&v=r3yVyt2gWBY
If you want to discuss details of the mods, let me know.
Cheers
John