As clubby says you need the specific tools for that Headshok, Cannondale seemed to change the Castle tool with each new model of Headshok/Lefty. Unless you know which tool you need it's a nightmare finding the right one, then there's the price
. A genuine Cannondale Castle tool for my Lefty cost around £80, but I was lucky in getting one for £8 in Edin Bicycle Co-op New Year sale several years ago. I also have a supposedly "universal" Castle tool bought for £25 from a Cannondale parts seller that definitely didn't fit every Headshok I have. Servicing the bearings on the slider also requires special tools but they can be bodged with care. The slider and bearings must be disassembled very carefully as each of the bearing races that the needle bearings slide on must be kept on the same faces of the slider/steerer as they have different thicknesses to take account of machining tolerances. The bearings will need serviced since the rubber boot is missing so they will without doubt be dirty, using fork without doing so will kill it.
I rebuilt a Headshok Fatty Ultra around 3 years ago, a complete rebuild including taking apart slider and it's bearings and races. Didn't service cartridge because I had a brand new one, once again Edin Bike Co-op sale came to rescue. Headshok went from 70mm to 80mm travel just like that.
As I took it apart I laid out each part in a marked area on a big sheet of corrugated cardboard so I knew where each part went for rebuild.
Marking the races and where they go on the slider/steerer with something that doesn't rub off will help in case they do get mixed up. It took me a few days of concentration and extreme cleanliness during rebuild but in the end I had basically a new fork. All Headshoks and Leftys are totally rebuildable with correct/same thicknesses of races if existing ones are corroded or worn.
This 4 page thread might help you decide which option to take, the DIY route or spend the money and let someone else do it.
https://www.mtbr.com/threads/super-...reassembly-step-by-step-my-experience.140972/
The fork has a spring with an elastomer and no damping cartridge but the disassembly of slider/steerer shows the bodge method of a short length of inner tube with valve and tape which I copied carefully and it worked OK. Don't attempt to bodge a substitute for Castle tool because a slip of bodged tool will result in damper shaft being scored, then it's new cartridge time, big money.