General Grievous' combat speeder is one of the easiest set to mod (modify) into a Star Trek like ship.
Extra bricks needed for engines, connections and bridge - saucer/bridge design is up to you : )
Minimal Variant
By itself, just remove the engine pod, flip it back to front and it looks like a Trek ship with mini wrap engines. Albeit with a saucer cutout - so just fill it in with spare bricks.
Build it a more elaborate engine too if desired.
Lastly, maybe rebuild the secondary hull and make the saucer (primary hull) detachable.
Excess Variant
If you've 2 sets of the speeder, cannibalize the 2 speeder engine pods. Use them for a single Trek ship's wrap engines : )
For reference below are my MMM LeoCAD LDRs for the Trek ships and the converted LDR for the speeder. The latter courtesy of EuroBricks Kuramapika1's LXF.
If you can't get the Lego speeder set anymore, there seems to be a WUHUI clone.