I really don't understand why Hobie wouldn't provide you a copy of the MSO, particularly if you can provide them a copy of your original bill of sale. As I said above, I've had other kayak manufacturers provide a MSO on used kayaks I've purchased.
You mentioned ACK. If you are in Texas and need to register your kayak because you're putting a motor on it, perhaps these instructions from TPP might help:
https://texaspowerpaddle.com/wp-content ... ctions.pdf There is also a link in those instructions to the TPW requirements.
FYI, some Texas counties now offer vessel registration at their regular taxing offices, saving you the previous requirement of going to a TPW office. I've never had any issues with TPW; but I know of others that "shopped" TPW offices until they found reasonable cooperation.
Personally, I'd take all the required paperwork for a used kayak, the original bill of sale (showing you paid sales tax), AND the Hobie MSO email to either your county or TPW office.