There is a set of numbers stamped on the down tube at the base. The first 2 numbers after the letters is the wheel size followed by possibly FX and 2 numbers which was the year of manufacture
The problem to me judging from your photos of the magnet is simple. Turn the bike upside down like has been suggested and give the magnets, ring and sensor a jolly good clean. Free, no cost, and simple to do as a first step.
Whoa! Not so fast. This cold weather reduces battery miles by up to half, same as an electric car. I myself am down to 30 miles but in the Summer it’s 50 miles easily.
wait a couple of months before you write the battery off or get more miles.
Found it, mine is a Ridge brand with a shoulder strap, but seems to be only 75 x 67 x 43. Have a search on eBay I would think there should be different sizes.
I got one a while ago from Halfrauds which was big enough. I could try to locate it a measure if it helps as I never use it now, but it must be in the blanket chest somewhere.