1960 Onan 305 CCK Quit Running - Hmm

So my fairly-new-to-me propane-fueled 3.5kw Onan CCK was purring along nicely last week when I was test running it for fun. All of a sudden it just up and died after about 20 minutes of run time. I pulled the spark plugs, which revealed there was no spark. Where do I start looking?