Welcome to the site
The air flow meters for the s12 and s13 are both the same as far as i know and im curious as to why you want to fit the s13 gearbox? The s12 gearbox is fine for 250 bhp easily and very durable in my experinece as ive not experienced any problems as of yet.
The s13 diff can be made to fit by using parts from both diffs.
A good move if you have an s13 is to fit the ca18det from it as it is a good base for big power and has alot more off the shelf mods available for it.