Ended up making 2/3 of a system from scratch, however; the list of improvements were:
* replaced the old screen with a better one. Also gives better battery life!
* used NES dpad and board so works like a NES controller now
* no ridges over the button areas now
* shoulder buttons work evenly
* larger speaker for better audio
* fan in center of system so virtually silent now
* provided a relocated memory cart port as his memory card failed. Easy to open system and put card in as a result.
Took quite a bit of work.
(note the relocated memory card port - there is plenty of space there for a card to be slotted into)
...and i've put a diagram on there to show which way up to put the card.
System is easy to open up, only held together by two strips of duct tape. Fast to open.