It would depend on a lot of things. For instance do you want to keep your clutch. How many tools to you have an access to. What kind of chainsaw motor do you have,. pictures of the motor and clutch assembly would be good.
I mounted a wheel on the drive shaft of a 42cc poulan and it worked fine for a while then it broke the drive shaft off from the vibrations. Since those experiments I have come to the conclusion that you need to build a friction drive unit which would connect to the engine with a chain. Then just weld a sprocket over the sprocket that is on the clutch bell. That's what I would do it I were going to do it today.
If you had access to the parts you could use a pulley and belt system as well. I never had those laying around so I never experimented with that.
I think I would use the smallest E scooter rear wheel, complete with sprocket assembly, as the drive. They make an e scooter that kids stand on. Then I would weld a scooter sprocket to the clutch sprocket on the chainsaw and mount all of it to the bike.
You can also remove the clutch and use the cutch parts to hold a friction drive wheel directly to the drive shaft. That's how I did mine. If you go friction attached directly to the engine ONLY RUN SMOOTH TIRES. Vibrations kill those engines. I know from experience.