You put a new motor in your 1989 Yamaha 125 dirt bike and it starts but when you put it into first gear it's not engaging any thought?

If it is brand new, it shouldn't be in the clutch. I would start with the basics. Check the clutch cable, make sure it's the right one. Check both adjustments on the lever and the clutch itself. If the engine is somewhat new, you can usually get away with replacing the clutch plates, spacer and springs. Don't buy the entire kit, they will try and sell you many parts that don't need to be replaced.

If it has a bull taco clutch, the clutch will be on the right side. Make sure your barrings haven't locked up. Maybe you haven't given it enough gas, so try a hill when you get going fast enough let the clutch out slowly and give it gas.