This is quite related to this topic
viewtopic.php?f=2&t=1101&p=16648&hilit=6th#p16648 but the how to do it, is simple: is with GP2Edit, exporting changes to your GP2.
First you select "Selected Team Power" option to use your selected teams' engine horsepower. (in GP2Edit it's Performace > Car Settings > Selected Team Power.)
Then, you choose to play the original GP2 carset or customize a new one with the hundred options available here. Each drivers grip can be changed, so you could also play the original 1994 carset but setting Schumacher a little higher than the 16384 he has.
By playing the original carset, I think a big challenge would be to play Verstappen, because you would race against Schumacher (grip 16384) with the same horsepower, rather than the +30 hp the game "naturally" gives you.
I still have the idea to do it, but I'm in a customization "mood" now. Last season I was able to score more points than a teammate with grip 16184, although he scored more often than me. (Having low rear wing set-ups is great for hotlapping but no so great to win 50% race distance. Those people who said in the internet "Oh, in GP2 you just put Rear Wing 1, a small change in the gear ratios and you're very fast" were not about long races.
I'm doing a third season now where I'm reproducing the conditions of the original game. I have the best engine and I have a teammate with grip 15984 (same as Hill in original GP2) and a rival with grip 16384 and -30 horsepower (same as Schumacher in original GP2). I'm supposed to win the championship with ease, because I understand 50% race distance set-up way better than in the late nineties. (I can share the car set data here, later.)
If I'm successful, next season I'll have a teammate with 16384 and let's see if I'm able to win the championship.
(You could also place Ayrton Senna in the original 1994 carset, give him grip 16384 and try to beat him in the other Wiliams. Or even better: place Senna and Schumacher at Williams, both with 16384, and try to win the championship in a Ferrari, with the +20 horsepower you'll have.)