I see the two nacelles on the TOS Shuttlecraft as quite large in proportion to the overall craft size. Could they make a warp field over the ~24 foot craft for a limited duration. Proportionally compared to the Enterprise, the nacelles-to-craft size is similar, so, why not. The only thing the Shuttlecraft lacks is large volume for fuel storage and/or a M/AM reactor plus support equipment like antimatter fuel production and matter fuel collection/processing. I do agree that the dilithium crystal, M/AM reactor and its closed system fuel cycle is the pinnacle of engineering in the TOS Enterprise and probably as small as it can be, so, no M/AM reactor in a shuttlecraft (yet.) YMMV by a lot. 
