Ah, somebody's doing the hair raising two speed calculations. Use this chain length calculator and remember to set the proper pitch--
Sprocket Center Distance and Chain length Calculators
Here's the deal on this--you'll never get the two chains absolutely perfect AND an idler will NOT work because that extra link or two will cause one chain to start skipping cogs. What you have to do is calculate several distances center to center from the PTO to the jackshaft, then calculate chain length with the sprockets of choice. You may have to adjust your engine plate forward as a result. You want to get within 0.004 or less (with a #35 chain). See this starting at post 325 from my bike last year:
https://www.oldminibikes.com/forum/...rs-lil-indian-model-600-vintage-class-33.html
Awesome project and I'm glad I caught up on this one.:thumbsup: Glad to help you if you get stumped on the chain calculations.