You probably Googled for the link you shared, but it includes an Amazon affiliate link (so someone makes money off people using the link). Can you edit your post to use this link please? It's the same link but with the Amazon affiliate part removed. Thank you.
https://www.amazon.com/Electronic-measuring-millimeter-conversion-measurement/dp/B094ZDBBZS/
What a bummer situation!
If it were me, I would probably try getting gasket material and making a custom gasket. I’m not sure if cork or gasket sheet would be better for you application, but they’re both cheap:
Allison 9732 Cork Gasket Material 1/16"x12"x36" https://www.amazon.com/dp/B000KKJQAU/ref=cm_sw_r_cp_api_i_9J733CZ37QC144DGKCB6
https://www.amazon.com/dp/B000CNISM2/ref=cm_sw_r_cp_api_i_0GFJ59VT2Q8J7MK88JA7
I would add a thin coat it in RTV/silicone when installing it, either way.
Glarks 6 Sets 2 3 4 6 8 12 Pin Sealed Gray Male and Female Auto Waterproof Electrical Wire Connector Plug 22-16AWG Connector for Motorcycle, Scooter, Car, Truck, Boats https://www.amazon.com/dp/B07L9XNFGG/ref=cm_sw_r_cp_api_glt_fabc_ZBNNGBWAVVSZYFJ3BH3C
I’ve used these before and they’re pretty good
You can buy boots that slide on or get spark plug wires
You need a battery to start and run your bike it will need it to have spark
You've got a couple options, but (as I'm certain you're aware of) you're biggest limiting factor will be your budget.
You could refurbish the exhaust: clean it, polish it, paint it etc.
You could replace it with something like this.
Or maybe you could see what a local fabrication shop will do, and then see what muffler suits your build.