I think I have seen them on ebay but I think any brand will work....they are just magnets. 
Try Area 13 for help.  They do respond quickly by email.  Good Luck.

You can have your old battery rebuilt.  Several re-builders listed on line.
My plan when the battery needs replacing is just to buy new WIRED bike. 
Batteries aren't cheap. 

Are you wearing sunglasses?  Some sunglasses are the issue. 
Also have you tried different angles? 
I have a Bolton color screen and have few problems with it. 

try a local bike shop.  they should have the washer you are looking for.
when i ordered a stepped up motor it did not come with a washer.

what kind of water problems are you having?  they are made to be water resistant. 

On Youtube one guy in California wraps his in plastic, but i would think that might allow the heat to build up.

you will always have that problem with RAD.  you will need to upgrade the controller - vendor Area13 to unleash the extra power.   they are also very helpful by email.  they will give you the controller settings that will give you the best bang.  i also upgrade my motor.  i peak around 1100 watts.  much easier on hill climbs.

with the stock motor that is about all you will get  the motor from what i have learned is actually a 500 watt that peaks at 750.  i upgraded my controller and motor and i peak around 1100.  Area13 is where i got my upgrade parts.

buy a different brand like WIRED bike.  lots of power and much bigger battery.  $2000.  sell what you have and buy the wired.

yes my error....chain

WD40 works great for cables.

Nothing wrong with being short.  But my understanding is that 22" tires are not common.  You will have problems with tire replacement.

I had an issue with a dirty chain.  It would click.  Once cleaned no more clicking.

Maybe an aftermarket controller will solve the problem.  Check out Area 13 for Rad controllers.

It wasn't difficult but it had it's challenges...mostly pulling a wire or two through the frame.  It was 3 years ago.  I think it is more of a pain to remove the rear wheel than the controller.  I have a Rad Rover 5.  Maybe the newer models are different.

probably power settings.  hold the upper and lower buttons for a few seconds to access the settings.  you might reach out to Rad for the correct settings.  also check youtube for how to change and access settings.