What is the thread pattern used on YOUR PTR-91?
IS it 5/8"x24 TPI the standard used in the U.S. for most .308 muzzle devices on .308 ARs and also used for some 6.8 and 6.5 guns?
is the thread the metric 15mmx1mm pitch that was used on the German HK-91s and PTR-91s before two or three years ago?
What muzzle devices you can install depend on what the thread pattern is.
IF it is brand new it is probably the 5/8x24 like the one I picked up about a year or year and a half ago but you will want to verify before you spend a lot of money.
(Look on the bright side, it won't be 9/16"x24LH like the FAL)
As long as you aren't in a state like CA that has restrictions on the cosmetics of the gun you can put just about anything on it as long as the threads are 5/8x24. You may wish to use either a crush washer or a solid or a peel shim in order to get the timing correct.
Things to keep in mind: Many brakes raise the noise level of the rifle. Although brakes are often touted as reducing recoil impulse and muzzle rise as a rule the actual effect on recoil is minimal. You can spend as little or as much as you want going down this path.
NOTE: I am linking to some vendors who are selling some of the brakes I've used in the past ONLY for illustration.
I am not endorsing ANY vendor. You MAY well find a better deal at a different vendor!
There are literally HUNDREDS of different styles and options and the price can range up through multiple hundreds of dollars if you feel the need to throw money at a bit of bling. The biggest reason I use the above items is normal inexpensive flash hiders are prohibited in CA. IF not for that my PTR-91 would still have the factory muzzle device on it. EVERYBODY makes them and they are mostly fisherman lures. Lantec Dragon, Griffin Armament, VG6 Epsilon, Battle Comp Industries, BCM and many others have been popular but unless you are looking for something that can act as a suppressor mount they are all mostly cosmetic.
ONE more thing to keep in mind that may not apply in this case but can be important.
IF you are installing a stainless brake on a stainless barrel use a little grease OR antiseize OR maybe even locktight as a lubricant.
Stainless threaded joints can cold weld themselves together in short order. I once saw someone installing a Battlecomp on a SS barrel and they got it about halfway on when it gave a tiny screech and stopped moving. It was cold welded to the barrel.