Need a 45 1-20 and got a Knight already?
 

Well if you got the other parts you can now buy the Mountaineer 45 1-20 barrel kit from Knight for $339 and about $16 in shipping
Its listed under Muzzleloader Barrel Kits at Knight Rifles website. Get a Mountaineer plug from Lehigh and it will fit some other Knights too such as the Extreme with very little work.

Mountaineer Muzzleloader .45 Cal 1:20 Fast Twist Barrel Kit

The Mountaineer Fast Twist Barrel Kit is a great choice when you need a new barrel for your black-powder rifle. All Knight replacement barrels are 100% American Made. Get yours today!

• 27” Fluted Stainless Steel
• Cantilever Lug
• Fits: Mountaineer

Would a disc elite or extreme bolt and breech plug work with that barrel?

Bolt will work, Elite stock will work. It already has the forked lug cutout.....but you need a Mountaineer or Ultra Lite breach plug. The plug is available from Lehigh Defense for $24.45 shipped with 1 vent liner. Another $14.50 if you need the bare primer adapter for the bolt.

Extreme stock depending on how new will likely need minor material removed in the lug area. New Extreme stocks should already have the cutout.