Any 2-stroke 125cc motocross engine such as the Yamaha YZ, Kawasaki KX, Suzuki RM (rare), TM motocross (different from their well-known K9/KZ10 ICC engines), or the ubiquitous Honda CR125. From any year (although one org. specifies models no earlier than 1991). Mods allowed.
In addition to the ICCs, they may even allow the older rotary-valved Formula C motors.
Usually, the class is restricted to 2-stroke 125cc motors; no 4-strokes like the Honda CRF. But, certain orgs. and series may allow them (I don’t know of any however). They might also allow for 80cc 2-strokes with a weight break. But 80’s are much rarer now than they used to be.
Since the Stock Honda package fits in the 125cc motocross engine category, it’s legal to run in the class as well.