Some engine manuals do describe a procedure to follow on first running and break-in. My first gas engine manual stated that I should run 2 tankfuls on the ground and then fly. My second engine didn't come with a manual since I purchase used, but I followed the same procedure with no problems. I ran the engine on the ground while mounted on a plane, but without the cowl installed. The 4-Star 120 should be ok since it doesn't have a cowl anyway. The first tank I ran at slower engine speeds and gradually increasing to approx 4000 rpms. The second tank I gradually increased engine speed to max while checking that the engine is not overheating or leaning out and backing off the throttle to let cool down before stepping up to the next higher engine speed. You don't have that ability if you fly the plane immediately.