Kind of a lot to unpack here
Sounds like you sorta skipped ahead on the whole build, assuming that the bottom end and everything else was sorted out. 120 might be a little on the lean side but I don’t think its way out of line. I would probably started at 140 or 130 and worked my way down but those are OKO sizes and I don’t know how the polini runs different.
I would start by pulling the top end back off, cutting new gaskets, checking the ring and piston. If the ring is stuck in the piston groove, you can probably get away with replacing it and sanding the piston and be fine. If you don’t pull it apart, you risk trashing the whole kit from the ring not making a good seal. If the ring doesn’t break in correctly and seal up, the combustion gasses go around the piston and heat it up and it will trash everything. Even if the piston is a little scuffed up chances are you can sand it up and pop in a new ring and save it.
If someone was bringing it to me like this, I would want to go through all the gaskets and make sure there are no leaks. Also keep an eye on the intake manifold bolts, you don’t want to over-tighten them. I use gasket sealant (motoseal, etc) to seal up the reeds, and blue Loctite on the intake screws and just barely get them snugged down. If you overtighten them it will warp the cylinder and that can cause a seize like you are describing.
Once you have everything back together make sure you double and triple check the timing. That will have more impact on it seizing than almost anything else. For breakin and tuning I would set it up like 15 degrees, super conservative, and as you get the carb close, then you can bump it up.
As far as the gearing goes, that doesn’t look like a 52 rear in the pic. That looks like 43 or 45, just eyeballing it. Use marty’s gearing calculator to set your top speed (guessing like 52) at 9000 rpm for that pipe. Then gear accordingly. My guess would be 20x45 not doing the math. The 19 will be close enough to get you tuned but you’ll get a bit more top speed off a bigger sprocket. Those circuit pipes do not rev out much past 9k. 10 maybe if you are perfectly tuned.
You really need to check for airleaks. And did I mention to check for airleaks. Also. CHECK. FOR. AIR. LEAKS.
The jetting you are talking about on that setup should never ever seize, shouldn’t even be close with that pipe, you either fucked up the intake manifold bolts getting them too tight (search for cornped/Maize’s youtube video on polini intake screws) or you have a huge air leak.