After messing about taking bolts out, measuring, putting them back, I have a list of sizes to dress up the engine bay. In case anyone else is looking for the info:
Bolt A - Wings & Bumper
23x M6 countersunk bolts, 15mm length
23x M6 countersunk washers, 19mm o/d
Bolt B - Slam Panel
2x M8 countersunk bolts, 20mm length
2x M8 countersunk washers, 25mm o/d
Bolt C - Airbox and Noise Maker
2x M6 countersunk bolts, 35mm length
2x M6 countersunk washers, 19mm o/d
Bolt D - Bonnet & ABS Unit
6x M8 flanged hex, 20mm
4x M8 flat washers
That will give you most of the OEM bolts around edges of the engine bay, plus the bonnet. I've not changed the ones on the pulley covers, or throttle body as I'm replacing these separately with something aftermarket, so they're not included in this list. Neither are the ones for the strut braces or the suspension, they're in good nick on my BRZ. I may do some more in future, I'll add any further specs as I find out.
For reference to off the shelf kits, you can put this lot together for about £85 from pro-bolt. Though this list is not quite as long as some aftermarket kits.
Some Of The Bolt Locations
This is all for info only, don't replace strong steel fittings with soft ally or cheapo items, stay safe