The replacement of parts is often noted in an aircraft lease agreement, as such the lease agreement will stipulate what can and what can not be used, on occasion it will also refer to a location and or part of the aircraft also.
The expense would be addressed by the lessee and statement such as “Lessee, at its own cost and expense, shall promptly replace, or procure the replacement of all Parts which may from time to time become worn out, lost, stolen, destroyed, seized, confiscated, damaged beyond repair or permanently rendered unfit for their intended use for any reason whatsoever”.