1986 WR400 Ohlins rear shock spring replacement

Happy new years fellow left kickers! My rear shock spring rate is not enough for me when I'm fully suited up and my desert tank is full (roughly 300 lbs.). I've bottomed out two times this last week end in Ocotillo and the rubber stop is half way gone. The rear spring has lost most of it's yellow paint and I know there is usually two different colored markings on the spring that indicates the spring rate. What spring should I have and can the rubber stop be replaced without completely disassembling the shock and losing the charge and oil?

The shock will most definatly need to be disasembled to install a new bumper rubber.