The Bulgarian AF MiG-21bis' have participated in quiet a few joint NATO exercises mostly with/against squadrons from USAFE flying the F-15 and the F-16. In open BVR it's nothing but a target drone as it has zero BVR capabilities, especially in head on. Whatever you do you need to bite your target on the neck from behind. But I've read some interesting comments from the US pilots who admit that the 21 is a nasty adversary in a hide and seek scenario. It has very small RCS and an effective terrain masking makes it very hard to detect. These tactics have been successfully used in penetration scenarios and in number of cases the 21's have managed to sneak in undetected. Though they've had to be completely silent - make your presence known and you'll be in trouble. The MiG-21 just can't survive on it's own in a modern air combat due to it's obsolete sensors but it could still be a though challenge if it's used smartly. That involves either AWACS or GCI to give to the 21 pilot situational awareness of what's happening around, most likely above.
So for sure it will be a challenge in the DCS world to be successful with this legendary airplane. You'll have to give it a lot of thinking and planning before you step into combat. Should be great fun, just can't wait to fly it!