| This may be easy if the new box will be in the same bay as the existing box. Can be difficult if not. I will assume the existing box is in line with the new location.
Once the new box opening is cut, snake (fish) a new cable from the existing box to the new opening. Depending on the amount of insulation there is will dictate how difficult this will be. may need to use an electricians fish tape.This is made of stiff metal or fibreglass to push thru insulation.
The new box for the paddle fan is important. It must be rated to support a ceiling fan, and must be able to attach directly and firmly to the wood joist/rafter. There are several on the market that are designed for old work, some metal with expanding bars that dig into the wood framing member, and some that are plastic that attach to the bottom of the framing member.
You can find round metal finish blank plates to cover the junction box where the track light was taken down. | 
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