Based upon the information provided, the requirement is for Aircraft Maintenance Services. What is not provided is if this requirement was determined to be commercial or noncommercial. If it was determined to be commercial, then FAR 12.207 and DFARS 212.207 will apply, which only allow a few contract types for commercial requirements; FFP, FFP-EPA, T&M or LH. The DFARS limits when T&M and LH can be used for DoD. 

Nonrecurring tasks are inherent to any kind of aircraft maintenance, and can fall under Over and Above Work IAW DFARS 217.77. DFARS PGI 217.7701 has the procedures for Over and Above and explains what it is about. Apart from how to classify these nonrecurring tasks, pricing should be fair to the work involved, to include allowing G&A and Overhead depending on the contractor's accounting practices. Case in point. If a PWS requires a contractor to acquire parts for nonrecurring tasks, then there most likely will be labor costs associated with the procurement of parts, labor costs incurred by those not performing nonrecurring tasks, and therefore will fall under Overhead or G&A. 

DAU offers various live events where you can ask these kind of questions. CONnect LIve is one such event, more information is at https://www.dau.edu/events.