I usually match donor visually and try a few until I find a working one, if I have them in the shop. If I am to order a donor I take into consideration the matching rules on here:
https://www.donordrives.com/blog/matching-guideIt should have the same number of heads to be able to read all data.
First you should inspect original heads under microscope for contamination / scratches / bents / condition of the flaps etc.
If magnetic dust is present on one of the heads you should try to clean it, install back, spin the drive a couple of times than remove and inspect again.
If needed platters should also have to be removed and inspected for media damage.
Only if condition of the drive is assest as acceptable you should install donor heads. Otherwise you may ruin them for nothing, which I don't do.
If your donor drive cannot start ok after moving back in donor heads it could mean donor heads just got ruined by the patient drive or you could have damaged them while swapping due to lack of experience.