Pilot flying CTSW ultralight aircraft reported engine malfunction after takeoff and made an immediate turnback to landing.

Narrative

Engine started and then stopped for trouble problem with flight display. Then ran checklist with engine run up. A Cessna 172 had just taken off. After Cessna turned crosswind; I started takeoff. At 300 feet AGL engine rpm dropped significantly. I immediately turned left for the downwind. The engine rpm increased; then alternatively dropped and surged three more times; then stabilized. I was able to climb to 1000 ft AGL; announce on the radio I was returning to land on Runway XX; and landed without incident.Once my engine developed problems; I lost situational awareness. All I could do was 'fly the airplane.' I probably flew in front of the Cessna. I am not sure what else I could have done. Flying straight ahead was not an option due to trees at the end of the runway (my flight instructor and a student were killed several years ago trying to clear the trees during takeoff with engine problems). I had pre-programmed myself to turn downwind should that situation ever occur to me. After landing; I was approached by two bystanders who told me 'I had flown close to the Cessna.'Afterwards; I discussed the engine issue with two A&P's. They had read about problems with Rotax engines using car gas with winter blends on hot days. It was 91 degrees [Fahrenheit] at time of takeoff. [This area] doesn't change to summer blends until Date. In theory; fuel vaporizes in the fuel lines and causes the disruptions I experienced. My airplane will be examined by my A&P next week. I will not fly it until cleared by my A&P.