Fooling around with your gearing first check is to rotate your front sprocket on the bike one turn and measure how far your track moves ( track run out ). Most of the 300 riders like 13.5 inches give or take, lower gearing than the bigger bikes.
The chain case idler from TS is just a wild guess, on all the kits I had to make a new idler arm to get correct chain tension.]
16 top 18 bottom in the chain case works to make a chain without a severe bend, and will give you the kind of runout you need.
Until the ARO, all the TS kits had the same length arms attaching the kit to the bike( except for their casual mtg. control and some kits would fit and others no way with stock chain ) and many models of bikes were problematic fitting the drive chain, on my 300 friends back in the day, they ran 12 on the bike until they had a near worn out chain, then could run the 13. 12 tooth with 520 chain is big power sucker, 13 is marginal.
I don't have my notes in front of me, I will try and find where we ended up on the 2 stoke KTM.
