I built a KV 122 a few years back.  I tried to to use a Frank's KV 85 muzzle brake for the KV 122, and drilled out the inside diameter to the 122mm barrel size.    I also turned a barrel  for the KV (using a 1/35 scale model, and mathematically converting it to 1/16).  

You can see the results below.  In both cases, it was like bringing a bologna sandwich to a buffet.  I thought it was great, until the real stuff showed up...  Now that Tamiya is building the JS, I'm going to be refitting the KV with a Tamiya barrel and brake.  I'm just waiting for them to have barrels back in stock, because they appear sold out at the moment.

My advice, save up the money, and buy the best...the first time.  Tamiya seems to have the shape and scale right. 

Muzzle brake is on the "E Parts", part #9000485.  Fortunately, both halves are on just one sprue.

Here's some pics though...  Tamiya JS-2 on the left, KV-122 on the right...

Frank's muzzle brake on top...Tamiya on the bottom.