Man, haven't seen one of those in a while, one of my friends used to have one back in the day. They were pretty rare even back then ('69).
Likely the rubber part comes backwards out of the side cover hole, of course, back then it was young enough the rubber was still live and very flexible. Battery may need to be out, and possibly other small things like rectifier assembly or harness. Is there an access port under the seat if removed?
I used to play with very small Yamaha twins and exactly the same but those split in a Y outside of the body hole and you really had to fold them up to get them through the much smaller monocoque air cleaner boot port there. 'Monocoque' is the construction type of that pressed sheet metal 'frame' there, like aircraft use.
If you can find an old Clymer manual for 175-200 cc. Honda twins pretty sure it includes that model in it..................you'll likely have fits finding any parts for it, at least for the engine. A few CB160 parts may work.
I'd be looking for SS125A rather than CB or CL125, you'll be getting mostly false hits there, the CB and CL were singles made at 25X the production number of the twins. The twins were popular in Japan and the developing countries. Most people here stateside have never even seen one. Like early 160 and 175, there may be more than one head casting with either one hole or two for carbs.