Re: Use current bicycle gears??
Check your local laws before removing the pedals. Some jurisdictions require them or they will call your bike a motorcycle and will require motorcycle registration, insurance, etc.
Do some reading here on the Harbor Freight engine conversions. Use the search feature or check the DIY section for non-kit engine installations. I feel you have a lot to learn before attempting this installation. Good luck.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"