Yz450f 2018 wont start

Hi guys, my first post on here so hi, and thanks in advance for any help you may be able to offer.

I have a problem with my bike not starting, I’ll give you the run down on what I’ve done, you’ll quickly see that it’s caused by me ?
So I’ve got this bike road registered but the standard mx muffler is way to loud for Uk laws so I had a wr450 muffler knocking around from a 2008 bike. I modified this one to accept the 2018 pipe and bolted it on expecting to have a nice quiet bike.

However with this new muffler on I hit the starter button and the bike spun over about ten times before giving a massive back fire and a flame out the back of the muffler, I tried a couple more times to start it but no luck so I took it off and tried to start the bike again. The bike now won’t start at all even with no muffler on at all.

I have a good spark and fuel but no start. My question is Have I done some major damage to the engine now due to the excess back pressure of the new muffler. I don’t know what to do apart from check the valve clearances.
Any advice greatly appreciated.
P.s please don’t just tell me I’m a tw*t for fitting the wrong muffler and cocking my bike up, I am fully aware of this ?

Thanks all