Well that was a fun morning!
Gas definitely in the cylinder heads - pulled the spark plugs and slowly kicked it over a few times. I heard splooshing and smelled gas so I gave it a more vigorous kick (key off, kill switch off) and a big whoosh of gas spurted out the right cylinder all over the sidecar. Kicked it some more and some dribbled out the left, and more out the right. Plenty of gas in there! (Also the left spark plug had some gas on it, too.)
After getting as much of the gas out of the cylinder heads as I could, we put the plugs back in and checked the oil.
WAY too much fluid in there! A sniff revealed a hint of gasoline, but it wasn't as strong as it was in the cylinder heads - but given the volume of fluid and the fact that I checked it before driving it down there, there was no question it was gas in the oil. Thankfully I took y'all's advice and brought the oil change stuff, so we did an emergency oil change. Didn't take too long and it was only 100-200km early.
Oil changed, plugs back in, we kicked it over... It kicked just fine so I assume we'd cleared enough of the gas from the cylinder heads. Turned on the key, hit the starter... Some rumbling but no starting. Waited a bit. Tried again. Still no starting. Waited a bit. Tried kick-starting it - nada.
Finally we considered that maybe enough gas had leaked that it didn't have gas, so we switched the petcock to reserve and tried again. Eureka! It took a little finagling but it finally started that time. Ran a little rough for a minute or two, but we let it idle and cough back to life. Rode it a couple miles down the road and put some gas in - only 2.7 gallons. So at least not *all* the gas came out.
Haven't checked the petcock yet - wanted to get more gas in first so it's roughly under the same conditions/strain where it started leaking. I'll go out and check it in a few minutes.
But thank you all very much for the advice re:changing the oil! That was invaluable. I got a lot of info re: hydrostatic lock and fuel leaking into the engine via the Search function here - I'm very grateful y'all have shared your collective knowledge because I have no doubt that without it, I woulda had a VERY unhappy rig this AM!
Pictures later after I get them off the camera. And also pictures of my convertible top for the sidecar, too!