The first thing to do is to verify that the Plus' problem is indeed only with the power supply, and not with other parts of the analog board. If the problems are truly limited to the power supply portion, then substituting a third-party supply should be straightforward, although a bit messy. See http://68kmla.org/files/classicmac2.pdf for some hints on how to troubleshoot the more common analog board afflictions, as well as for pinouts that might be of assistance in performing the surgery you propose.

Good luck!