ypvspares.com is an excellent UK stockist of Tomos parts, and do some big-bore kits.
You're right about the two improvements being carb and pipe.
1 - Definitely get rid of the awful Encarwi and get a Dellorto carb (SHA is fine, but if you're also thinking of a kit, maybe a PHBG instead). Making an intake is easy enough: https://www.mopedarmy.com/forums/read.php?7,3666475,3666644#msg-3666644
2 - Techno Circuit or Bullet exhausts are both cheap and apparently the Circuit is really good for low-end power, which you'll need if you're hill climbing a lot.
A3ML means it's 26x22. I'm just about to put a 24-tooth rear sprocket on my A3ML so it can climb better. Might lose a few mph at top speed, but not crawling up hills at 12mph is better. Also going to open up my exhaust port to 28mm and maybe put some piston ramps in so the blowdown is closer to 17%. Modding the piston rather than the cylinder is a good way to experiment without turning your engine into a boat anchor.
Might be an idea to change your piston rings and re-time your points, too. Get a cheap timing light - A3's are very fussy at what RPM they make power vs. points timing.
Good luck, and keep us informed! ;)