GURPS Ultra-Tech p. 207 covers how to build cybernetic body parts. The short version is that you take the appropriate disadvantage (One Arm in this case) and then apply a Mitigator (B112) based on the limitations of the cybernetics
According to Sean Punch Unhealing by itself would be a -90% Mitigator. The usual combination for cybernetics given in Ultra-Tech is Unhealing, Electrical, and Monthly maintenance for -70%
Mitigator limitations are not subject to the -80% limitation cap since that's not exploitable the way it would be with advantages.
One Arm (Mitigator, Unhealing, -90%) [-2]
One Arm (Mitigator, Unhealing, Electrical, Monthly Maintenance, -70%) [-6]