[400TT]

Yes, you certainly can test and certify seats.

We certainly do and have certified seats for our kits.

ADR3/03 gives the testing criteria.

It's a little bit simpler in a 2 seater as the seats don't need to be tested for protecting the occupants from behind.

Anyway if you keep hunting you will find plenty of ADR approved seats around. Finding one that suits is the challenge though.

Also make sure it meets the headrest height requirements specified by your engineer.