I once accomplished a valve clearance check on a TL1000 (basically the same engine) with a badly stuck timing plug by putting the bike in second or third gear (can't remember which) and having a friend slowly rotate the rear wheel until the timing marks lined up. (Take the spark plugs out first, which I hope is obvious.)
It took a several tries, but we got it done. Later on, it occurred to us that we could have done the same thing with maybe a little more control by using a socket on the countershaft with the transmission in gear to turn the engine.
2002 Suzuki DL1000 V-Strom, Dark Metallic Space Blue
1983 Suzuki GS850G, Cosmic Blue
2005 KLR685, Aztec Red - Turd II.2, the ReReTurdening