Canada, putting frogs and maple syrup in the same sentence.
For the carb, you're better off keeping the OE. If the price of a quality rebuild kit is off-putting, you may want to consider moving on to another hobby. It's only going to get more expensive.
Tires aren't 'affordable' anymore, for anything. In the US, about 10-15 years ago, tire prices skyrocketed and never came down. If you just want a wider stance, you can upgrade to newer hubs that use a 4x110 pattern, then wheel choices will expand. There's no easy solution for the front and no bolt-on wheels available other than stock.
Thumb throttle, as it's just more functional for the way a trike is ridden. Motorcycle handlebars aren't actually turned much to make at turn which is why a twist throttle works well on them, the rider isn't hanging off the side of the bike while changing direction. Sure, road racers hang off the bike, but they are on smooth pavement. It's personal preference too, but not many ATV riders are switching to twist throttles. There is a dual throttle out there, twist and thumb, but it's expensive. For long rides, that may be nice.
For someone new at mechanical anything, the closer you keep it to stock, the less problematic things will be and the more you'll possibly enjoy it.
For better foot pegs, Wicked Metal Design has bolt-on extensions. I have some and they're nice. No fuss, just bolt on and ride.
https://wickedmetaldesignsllc.com/pr...0s-add-on-pegs
Besides fabricating something yourself or buying some insanely overpriced vintage heel protectors, those Wicked Metal pegs are relatively affordable and time saving.