I've done this on mine and several other SH's; Remove Nut, Counter shaft Sprocket, Seal Sleeve from shaft, O ring from shaft, I then used an exhaust spring hook tool and levered the old seal out. Being careful to not gouge the case/seal surface. Thoroughly clean the area where old seal was seated. I use Honda Bond or any silicon gasket goo to sparingly coat the outer edge of the new seal to lubricate for installation and seal upon completion. I put a new O ring on the counter shaft. What works for me is to insert the seal bushing/spacer into the new seal. This helps center the seal in the case. Slide the seal and spacer assembly on to the counter shaft. I use a large, deep old bushing of the correct diameter I had in the shop and Carefully tap the driver with a hammer until it's seat evenly flush with the inside edge of the slight beveled inner edge of the seal area in case. If you don't have access to a proper driver (just slightly smaller than the outer diameter of the seal and longer than the counter shaft ) I've used large drift (1/4", 6-7mm, etc.), socket extension, or whatever. I know people that have used PVC, ABS and steel pipe with success. You can do this!