loasd more info required for good answer!
Nothing better than having your own gear!
I wouldnt take a full set of gear! I would try sqeeze in a mask, set of fins and a snorkel. Perhaps fins that dont require booties?
Where are you going?? How many dives?? Loads of dive places I have been will throw in the reg, BCD, belt, tank for real cheap. Once you pack your bike with everything you need for the trip, then see what you can squeeze in.
Also on a trip of mine on bad roads I have had all sorts of equipment ruined from vibration. Would hate to think of what it would do to your reg and especially your BCD. I had to give up on the idea of taking only the basics when I ran out of space!
Also you didnt say if you were in a car or a bicycle? Are you camping?? Loads more space if you are not. Going alone? Let your buddies carry all the gear!!
In short if you are camping, on bad roads, alone, and doing less than 15 dive locations, then DONT!