Well, since we don't see Scimitars fighting WC3 ships in official games we can only guess. I don't think the old ships are much worse, because we know that some WC1 era ships were used even after the war.
So you might generally use WW2 rules to compare them: A P51D Mustang is better than a BF109E, still if they fight 2 vs. 2 the BF-109 has a chance to win. It is rather that than something like F/A18E vs. Mig 17.
Also the ships might have been upgraded during the years. The following ideas are based on the fact that it wasn't upgraded much or not at all. Also I guess you want to make a mod or something, so the balancing matters. I would even dare to say that the balancing and fun for the player is more important than accurate canonical numbers. I also used that approach when I gave the Dralthi IV and Darket more decoys because otherwise it sucks flying them in the Saga engine.
So here we go:
The Scim is old so maybe its shields are weaker. So you have to be more careful not to get hit.
It is not very fast or nimble. A Vaktoth may be a dangerous opponent in an un-upgraded Scim.
The approach Saga used for the old ships (the Sabre being the flyable one of those) to make them feel different was to have faster shield recharge but less shield maximum strength.
Weapons: The Scimitar has only two guns, most WC3 ships except the Darket (which has also two) have at least three or four. The range of the mass driver is also pretty limited, which gives the WC3 era Kilrathi ships another advantage. Meson cannons allow them to hit the Scim pretty early before it is in range.
It carries up to five missiles, two of them being dumbfires. The Darket carries only two, while the Dralthi carries four, so the missile load is comparable to the WC3 ships. (the Hellcat has six). For reasons of simplicity I assume that those are the same missiles for now, since I think you won't use 10 year old missiles if possible.
Anyway, with the shields the Scim has you should avoid getting hit by missiles. But you should do that anyway, a single heat seeker kills the Darket or Dralthi IV I'm flying as easily as a Scimitar.
So it is inferior, but not that much. You can fly against WC3 ships and beat them. Just be careful, it is not a Thunderbolt.
Shields and weapons are also interesting for balancing:
How many hits does a Salthi or Dralthi (two laser cannons) need to kill a Scim? Or better: How many seconds can a Scimitar's shields stand against fire from those? (Fire rates are different in different games, the WC3 laser cannon fires much faster).
So you might take that and try it out in your game engine. Just place a Scimitar playing dead and attack it with an Arrow's lasers to measure how long it takes to beat the shields down with lasers.
The Scimitar should be able to take more shots than a Ferret, but not as much as a Hellcat. I think I would approximately aim for the Arrow's shield strength. Perhaps a bit lower but a bit faster recharging. Its armor may be in the region of the Hellcat. You won't get an uber-fighter but you will live. Paktahns and corvettes may be a P.I.T.A. though
...now that I'm speaking of it... I should really ask Deathsnake for one or two of his models and do that myself. Killing Scimitars in my Kilrathi mod might be fun....