That is an amazing find Lewey, I didnt even realise they did a 3-door SX! It doesnt even have electric windows, which I thought were standard on the SX.. Has it got locking?
Not sure I`d want to modify it personally, it could conceivably be unique, but its your call.
Why dont you get a rust free late Mk2 shell & re-panel it as a Mk1?
There is a 5-door sill on ebay for £40 at the moment which will give you the front section of the door step as well as the full length of outer lower sill if you chop the B & C--pillar sections off, item number 300874148358.