Yes, it does sound like crank seals.. but here's another potential clue:
Search the buy/sell forum for a post from Chad, about two weeks back. He was looking for a replacement black-box for the transistorized ignition on the Jawa 210. Unless he owns another that I did not see, I assume he was seeking one for this same machine. Now, this could be a total red herring, but it's worth looking into.
Here's another thought: Go do another of your choke-on 30mph runs.. a mile or so. Then pull the plug and check it out. If the plug color is something reasonable (light tan to a chocolate brown), that would tend to indicate that the choked carb is providing a reasonable air/fuel ratio. If this is the case, you need to have the crank seals tested. Any shop that works on dirt bikes, or ATVs has the tools for the test.
If the seals are bad, you're in for some interesting times.