My limited understanding is, if the SP89 was imported after the '89 cut off for "non sporting" rifle importation (my understanding is that most were) then the SP89 would be subject to 922(R). So to convert it into a "non sporting" Title I rifle or register as a SBR, no more than 10 foreign made parts can be used in a "non-sporting" semi auto rifle. So whether that rifle is NFA registered or not it will have to be 922(R) compliant. So if the pistol was an original German 1991 import or a new made MKE pistol, they would both need to be 922(R) compliant to be a rifle. Would you really want to pull all those German parts out of it to be compliant? Good luck with your SBR project.