Yes, you can fit a 100/80-17 front tire on a Bajaj Pulsar 150cc UG4.5 without any modifications. The 100/80-17 tire size is compatible with the stock rim size of the Pulsar 150cc UG4.5, which typically comes with a 17-inch front wheel. However, it's always recommended to consult with a professional mechanic or refer to the manufacturer's guidelines to ensure proper fitment and compatibility.