Vintage Synth Repair

ok, new plan-i left the unit on for an hour, and the + rail seems
solid, but -15 drifted down to -12.5. both sides of x-former were in
the 23VAC range, so i have a new plan:

1-stick with old PSU
2-replace + and - power transistors.
3-socket and replace voltage regulators while im in there.

any other long term preventative maintenance? can i expect a
transformer to last another 10-20 years, or should i bother with it
now? parts cost isnt an issue, its well worth the investment.

thanks for the help, folks.