Probably years late-to-need posting this, but for anyone fighting electrical gremlins and chooses to replace the 40+ year old contactors/solenoids, Cessna SK 337-74 provides for replacement of 14V and 28V starter and battery contactors (solenoids).
It looks like the modern versions of the PMA Lamar SkyTec contactors are:
Cessna P/N SkyTec P/N
S1577-1 STS-S24 (#07-03563) 28V Starter for Front/Rear
S1577-1 STS-A24 (#07-03567) 28V Ground/Aux Power
S1580-1 STS-M24 (#07-03565) 28V Battery/Master
One source: aircraftspruce.com (search for STS-S24)
Other info:
Lamar installation guidance
http://www.lamartech.com/solenoids---contactors.html
http://www.lamartech.com/files/128985429.pdf