Try carrying all speeder parts on you at the same time, all copies of the repulsor or anything else, rather than swapping them out. Maybe you tried this already, but it looks a little like you tried only carrying a couple at a time.
I had this before and solved it just by throwing everything related to the mount (which included a fair few duplicates) into my inventory and trying again
Edit: I re-read and It sounds like you have tried this, but best to be sure.
Yeah, I did do that exactly that (every single Part I had, Exploiter, Expleror and Pleasure), took everything I had at once to the GSI Vendor on Nar Shardaa, still wouldn't hand over the Speeder. Very aggravating when it won't hand it over XD
Just found an Expleror in my inventory thanks to David, thank you David! Hopefully I don't have to go through this again when I get my final Pleasure Speeder part D: