To try and answer your question I would start by looking at the free downloadable Taka manual on this website, or purchase on the web. There you will find lots of the information your looking for. Oil for engine/clutch case is 30W. Fork oil is often referred to as "Hydraulic oil" in the manual. It can range in viscosity weights of 2W to 15W or higher. I use a light range oil (10W).
Not sure what issues your experiencing with the clutch. Often times you can pull it apart and gently clean and scuff off any surface glazing using common sand paper. The proper oil height in the clutch case is measured by the side bolt. Fill the case until the oil spills out from the lower bolt.
As for parts - you can try an contact 6X6 or Richards as indicated on this site. Also, Ebay has many sellers offering tons of Bridgestone / Rockford parts. You can also post a parts "Wanted" listing on this site.
Hope this helps in some way - Good Luck !
Kevin
