The muffler should not affect how the engine runs significantly, but you should repair it soon.
I have the same O-ring kit. I rarely have a situation where the O-ring does not fit, and generally by a small margin.
If they do NOT fit, you can cut the O-ring to size and use super glue to connect the free ends. Super glue works via an anaerobic reaction, so the two connected ends will stand up fairly well, even to solvents and gasoline. There are also glues/ adhesives designed for this.
I generally use Prinz 406
https://www.amazon.com/Loctite-Insta.../dp/B00B3HZPWC
That said, any Type II glue will work. It is the same glue used in 'cut to size' kits.
https://www.zoro.com/value-brand-sta...44/#tech-specs
81 250R Custom
83 250R (in progress)
98 Scrambler 400
03 Bombardier Quest 500
07 Polaris Predator (sold to klondike1020 )
68 VW Dune Buggy