Welcome to the forum. Glad you've joined and hope we can offer some help.
If you suspect a blown head gasket check for signs of oil under the head. There should be visible evidence as well as poor performance. 26 mph isn't bad for an engine that isn't fully broken in. You didn't say how long your "trips" are but you'll need to burn through a couple to three gallons of fuel, varying your speed from WOT to a slow cruise during that time. What fuel mix ratio are you running? 24:1 is okay to start but I like to see that reduced to 32:1 after a couple of tanks. What spark plug and what is the gap? These little engines run better the more miles they accrue.
Tom