85 kz250 air filter / carbie issues

G'day folks, first time posting.

Bit of a longer explanation for context before getting to my question, hope that's alright. I'm still learning bike engines so please bear with me!

I've been riding an 85 KZ250 Ltd belt drive for the last 6 months after having it looked over by a mechanic.

Recently, it blew a head gasket, so I took it to the same mechanic to get a replacement. He ended up needing to completely recondition the valves due to carbon build up, understandably.

Upon giving it back to me, the idle was all over the place, either screaming or dying. I took it back, and the fix ended up being sealing up an air leak around the boot between the carby and head. I was a bit unimpressed he'd given it back to me with this issue and initially blaming it on having an old carby, but the idle issue was resolved.

Looking over the bike once I'd got it home again, I saw a split in the crank box vent tube leading to the air filter, so I sealed that up and hose clamped it for good measure. Immediately, I found the engine unable to turn over. Upon removing the hose clamp, the engine turned over, which lead me to believe the split was allowing extra air into the filter and carby, thus balancing out the mix.

Obviously then I became worried that the split in the crank hose would allow dirty air to pass into the carby, (possibly being the original cause of the blown gasket?) so sealing it up like I had done earlier would be best in the long run.

SO, now the issue would seemingly be either the air filter needing replacement or cleaning, OR the carby retuned to let more air in.

I opened the air filter box to find a pretty dirty (probably 30+ year old) filter. My goal was to clean it up as best I could, but the filter wouldn't come out of the box, and trying to force it seemed to be causing damage, so I stopped for the time being, reinstalled the box, and moved onto the carby retune instead.

Upon reading up on how to allow more air into the carby, I found myself unable to locate the air screw. The idle screw was obvious, but I simply couldn't find another screw anywhere, which seems crazy. Either I'm dumb (likely) or this little carby doesn't have a visible air screw (unlikely)?

While the air filter box was off, I looked through into the carby intake and noticed the value was all the way open, suggesting that the carby was tuned all the way open anyway, which points to the dirty air filter being THE main issue; simply not enough air getting in.

SO, my two questions are these:

- how to I get the air filter out of the filter box without breaking it? (I've read that those original filters are near impossible to come by or even substitute)

- Where the hell is my air screw?

Photos attached for clarity.

Thanks so much if you read this far! Please feel free to point out any extreme ignorance - I'm ready to learn. Just want to get the most life out of my sweet KZ that I can.

Cheers