I knew there'd be one. - In all seriousness, the bucket seats will be easy enough if you can find (or make) the subframes to mount them.
The splitter will almost certainly need to be a custom job; if you speak enough Japanese, Crazy Hornet might be able to ship you something. However, I think people have tried and the orders just weren't big enough for them to ship to the UK (or too big to warrant just putting it in the post).
You won't find much help with purely cosmetic modifications in the owner's club, but if you're actually looking to improve the handling and safety there's a lot of knowledge.