fundreamer1, from seeing the picture I'd say that your chain size is a 420.
Suggestion's, check your LBS or online for a smaller pedal chain ring, and shorter pedal crank arms, if you are having clearance issues with the engine.
Remember you'll most likely have to run a small cog / sprocket under the right side of the engine for your pedal chain to clear the engine.
Make sure you have the engine drive sprocket lined up with real wheel sprocket before you weld in your skid plate / engine mount.
Check online for the swivel adapter for your carb intake if you have any clearance issues with the carb and frame.
Build looks good so far!
Have you started a build thread in the Stretch Cruiser & Chopper Bicycles, section of the forum?
Suggested titles for your thread:
1. OCC Chopper Pitbike Engine
2. OCC Chopper Lifan 50cc
Hope to see you starting your own build thread with this Chopper build!
Peace Crazy Horse.