04b IIRC is a 6mm chain and 25h works with fine (i can vouch for t8f and 05b 8mm chain). i take it a 13 tooth sprocket ( think that what stock on those motors) so that will require a 65 tooth sprocket (if your going for single stage) which will require a 126mm platewheel. https://www.gearsandsprockets.co.uk/...lex-sprockets/

or you could use a two stage system which will make it more compact

I fine THIS website helps with designing a gear ratio