I have had some experience with these types of units, altho not TVR specific by any means.
My first thought would be, that ain't all your problems!
A little more info might help.
Like, was it running just fine, bombing up the hillclimb, and jammed, or is this something you dragged out of 20 years of storage?
If stored, and you have moisture/corrosion issues, be prepared to find a rebuilder.
If it happened in use...and specifically backfired....might just be the plate not centred in the bore and jammed. Look for light around the plate.
I always hated those things..of course, I generally got them when they were brought in with emissions failure issues. And very high miles.
Dave