No there isn't. There are only 2 o-rings in a TS250/1 engine - gearchange lever & kickstart shaft seals in the clutch cover. There is a small rubber disc that fits between the crankcase halves, but this is on the crankshaft side & serves to distribute condensed two stroke oil between the main bearing drillings (item 6 on drawing 4.2.1 on page 94 of the factory manual).
Sealing between the gearbox & crankcase is down to flat joint surfaces & an apprpriate non-setting jointing compound (I used Blue Hylomar). If you are still getting gear oil in the cases (or fresh gear oil gets to smell of petrol quickly) after the crank seal change, then it is a case split to reseal the joint face IMO.
Given that your crank seals were shot, is it possible that there is an accumulation of gearbox oil in the bottom of the crankcase that is taking a while to clear ? Unfotunately there isn't a crankcase drain plug on this model, so a crankcase flush with premix & drain requires the engine to be "up-ended".
You could eliminate the air filter by just leaving it out for a trail run. Timing should be 2.75mm BTDC (mid point between the 3mm +0/-0.5 given in the manual).
If it would help I have several carbs & could send you one to try out as an alternative to yours ?
If it is any consolation, it took me 5 years or so, on and off, to get to the bottom of why my TS250/1 kept siezing - and even then I only found the cause by accident. I would have a session on it, try it out after modifications / interventions / checks, the put it away again when I got frustrated. The cause eventually surfaced after having had the carb off (again !) - when re-tightening the clamp screws, I found the carb would still rotate in the socket, so tightened the clamp a bit more. One of the lugs broke off. What I think had happened was that the bike had been run with the carb loose & the socket had worn, which then caused an air leak. A NOS intake manifold fixed the siezing problem. Not the issue you have having but, with "simple" two stroke engines, it doesn't take much to upset correct running. Finding the cause can be tricky.
Don't give up on it - just stand back a bit when you get frustrated, regain your composure, then try again. It is only a machine & recalcitrant machines can be fixed - eventually.