The stock exhaust clearly can't be used.
The S90s have the ability to go to 105cc with a big bore kit but you are having to change the carb as well as the exhaust. The carb is the easy part. Fashioning a custom exhaust is about the only way to do it as nothing close to stock fitment from other bike is easy... So, making the exhaust a two piece affair and doing a little porting work is needed to take advantage of the longer duration camshaft that is available for that motor. Some guys just skip that route and bolt up a good 140cc horizontal motor, that's a Taiwanese motor usually for better life and some of the aftermarket motors are quickly junk while others are reliable and well made and give good life.
The 100cc can easily drop into a C70 Passport and guys have squeezed a 120cc but clearances are really tight and you won't be using a legshield with that mod.
The Sym Symba is a nice little bike of 88cc capacity that is a modern Honda Cub...Sym built Honda parts for them for a couple decades so, when their relationship with Honda ended (I think it ended) they started building bikes for themselves. They make a high quality small motorcycle (tiddler, as termed by many small bike owners...)
The Symba is quite peppy and can easily and quickly do 50-55mph. It is a fun bike and it handles very nicely. I test road one after they came out...but, their 2500-2800 price is a little high, but getting 75-100 miles a gallon is very tempting if you live in a big city. The CB550 or a CB350/360 make for nicer city bikes, the CX500 makes an excellent commuter bike but it is rather boring, very good low end acceleration, just horrible brakes stock, But that is easily improved with a twin pot Honda caliper and a set of stainless braided brake hoses and appropriate sized MC or even the stock one gives good brake feel with the larger caliper piston area.
Dropping a 185 in a CB125 I would want to get rid of that mechanical front disk brake that is so problematic and either put a different set of forks or convert it to disk with hydraulic brakes...
Stopping is important and anytime you do that kind of hp mod, stopping gets to be more important.
If you do the upgrade, please create a thread on it...
The CA95 154cc motor's cylinders and reworking the stock head for the CA72/CB72 125cc bike to give it a boost, it was a Honda supported upgrade. The CA72 had twin carbs as the CA95 had a single powerjet carb on some models.
I forget the model now, but the first small bike motor that Honda developed that had 20 bhp was sometimes fit to the CB72 or even a CA95...they had a vertical/upright motor and one model that had the forward slant... The later was used in the upgrade...but then you faced the other upgrades. Was it the CB175? Been too long and it is too early/late to remember that right now. Migraine headache doesn't help...
I forget the member ID here who did several CB125 rebuilds...
He built some nice ones... should be in the Other bikes or Project Bikes area...but it is more than a year old.
David