Here's the bing next to another bing 12. I drilled it out to 14.3mm (9/16") and matched the intake side to the intake which measures 14.7mm. Polished the air box side.
I plan on gutting and smoothing the factory air box. Where the cylinder meets the intake will be opened to like 16mm, and intake matched to that.
Probably gonna split the cases and open the transfers up a bit and match them.
I have 13, 14, 15, 16 and 40,45 (stock maxi sprocket array lol) to play with for gearing once its together.
More things to add but my fingers are getting tired of typing lol.