I dug a fairly large hawthorn (5 m tall and 15 cm diameter) today from a
neighbor's yard (it got cut back to a meter). However, the roots are
terrible. How well does hawthorn sprout new roots from large cut-off roots
when transplanted?

From my limited experience, hawthorns are pretty tough. The only exception I
found was Washington thorn, which died after I put it in the ground. English
hawthorn did fine after I wrestled it from a nursery pot into a bonsai pot. I
had to cut off a lot of roots, but I also cut off a lot of the top.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)