Most likely 1”. If you have a caliper, you can easily check by measuring the handlebar stem, where it attaches to the fork. 21 - 22ish mm means 1”, 25.4mm means 1 1/8” (the stem is smaller diameter than the fork’s steer tube). If you don’t have a caliper, use an adjustable wrench and measure the opening with a ruler.
Triple tree will probably hold up better than a springer with a motor, depending on the forks you’re looking at.