The only codes you can bill are as you listed: 42120-LT and 38724-LT
The cheek flap, because it appears to be internal (mucosal), is included in 42120. If the surgeon had used an extraoral tissue graft/flap, then that would have been separately reportable.
Hope that helps,
Jennifer M. Connell, CPC, CENTC, CPCO, CPPM, CPMA, CPB, CPC-P, CPC-I