Ha ha I had the same problem, I lowered it that much I hit the drive belt with the top deck!!!!
I have no idea of the diameter for you box but obviously there must be a wear issue if you cant reach it very well.
Any Idea what make your starter box is? If not post a pic and I'm sure I/someone can tell you
As for the flywheel most Nitro engines are much the same besides some are 2 pin and some are 3 pin for the clutch shoes , I can recommend www.answer-rc.com or Apex models, fire them an e mail. Yours will probs be a 3 pin!
Answer RC defo stock them, and they look sexy! about 7 quid, you may wish to fire a set of new shoes in there a whilst you have the clutch apart. (age depending) they are about 8 quid for a set.