After seeing the lack of entries, I decided to come up with my own few days ago.

This would be a recommended field upgrade of the TOS shuttlecraft. The Enterprise was able to pick up a new complement of shuttles when she came in for the refit, but not all ships have that luxury. I figured that after the incident with the M5 computer, the Lexington would have come in for an extensive overhaul, after which she was sent on a long duration mission. So while other Constitution class ships were coming in for refit, she was still out on the frontier, during which time the plans for this shuttlecraft upgrade would come in (including current marking standards). This replaces almost the entire propulsion block, incorporating the newer impulse and warp engines of the later shuttles (which I'm assuming are more efficient and slightly faster), but doesn't waste the rather sturdy spaceframe. A result of this is that the newer impulse engines free up some cabin space, also allowing for a rear hatch to be installed. Also, the newer warp nacelles remove the need for the extendible landing gear.
I used several sources as a guide when drawing the original shuttlecraft, including some of Warped 9's excellent drawings. The drawing was done in Corel Draw, with shading done in GIMP.

This would be a recommended field upgrade of the TOS shuttlecraft. The Enterprise was able to pick up a new complement of shuttles when she came in for the refit, but not all ships have that luxury. I figured that after the incident with the M5 computer, the Lexington would have come in for an extensive overhaul, after which she was sent on a long duration mission. So while other Constitution class ships were coming in for refit, she was still out on the frontier, during which time the plans for this shuttlecraft upgrade would come in (including current marking standards). This replaces almost the entire propulsion block, incorporating the newer impulse and warp engines of the later shuttles (which I'm assuming are more efficient and slightly faster), but doesn't waste the rather sturdy spaceframe. A result of this is that the newer impulse engines free up some cabin space, also allowing for a rear hatch to be installed. Also, the newer warp nacelles remove the need for the extendible landing gear.
I used several sources as a guide when drawing the original shuttlecraft, including some of Warped 9's excellent drawings. The drawing was done in Corel Draw, with shading done in GIMP.