As far as I know, the first two stages of the Fisher Price gearboxes in the kit of parts use imperial DP gears. Currently, they use 32DP, but in the past they used 24DP. At least for last year's gearboxes, you can take out the first two stages and install them in your own gearbox housing and run them an entire season without any problems.
Here's a photo of a gearbox I designed last season to use the first two stages in an easier-to-mount enclosure to power intake rollers. The calculations were done using 32DP for gear pitch with +.003 added to all center-to-center distances for machining tolerance, and it worked just fine through countless hours of operation and five competitions.
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