BMW G650X (All) Forks
The 45mm Marzocchi forks come in a few variants for the models but are all very similar internally. Inside they are very cheaply manufactured, using a crimped steel cartridge that cannot be disassembled, and therefore cannot be rebuilt, modified or repaired. They are very similar to the units found on the F800GS and need the same kind of modifications.
Stiffer Fork Springs & heavier grade Oil
Not a perfect answer but better than stock. Stock springs are 0.48kg/mm, and we have rates from 0.44 to 0.85kg/mm in our 19-145 Series Fork Springs.
Marzocchi Shiver Conversion
Since Marzocchi went bust in 2015 the parts are too hard to get and too old to make this conversion viable for us anymore. We used take a set of Marzocchi Shiver MX or Offroad Forks with compression and rebound adjustment and graft them into the G650 fork. We'd change the fork springs from progressive rates to linear too, making the front end more predictable. It required complete revalving of the cartridge internals, delicate machining of the axle feet feet for the compression adjuster too, and lacked an external spring preload adjuster.
It was a massive step up from stock, but takes a lot of money and a lot of time and effort to collect all the bits and pieces needed, so we won't be doing any more.