I used to do this, I have "retro modded" Thinkpads. I keep one still running (X61p) to use as a car computer. I've since switched everything I do that isn't on my Macbook to a Framework 13 AMD. The Framework 13, other than the case design being flimsier plastic vs magnesium alloy, is better at everything a new Thinkpad does than a new Thinkpad, and is roughly equivalent to an old with modern specs. As much as I wish that Lenovo would make a modern equivalent to the X60/X61, that's never happening, so Framework 13 is the best choice right now.

Retro modded like you replaced the mobo and whatnot ? How hard was it ?