Some time ago I tried to simulate races with no pit stops. Although Roberto is right about editing the magic data, although I think it is better to make the first pit stop at the race length plus one, there are some problems.
1. The cc cars can find it very difficult, especially at certain tracks, to race with 70, 80 or even 90 laps worth of fuel.
2. Whilst tyre wear does not effect the cc cars it is impossible (I think) to make the human tyres last a full race length without changing.
3. The races are very processional, as each cc driver with the same fuel load just drives at their best speed with no overtaking. Try running a test race with all cc cars and look at the lap chart at the end. I can almost guarentee the race order will not change from the end of the first lap to the end of the race, barring pitting for mechanical problems. This in itself is a problem as a car with fresh tyres late in a race will be unrealistically fast.
Don't forget that for much of the 80's there were pits stops but for new tyres only, but they were much slower: about the length of modern pit stops with re-fuelling. I am afraid I cannot see a way of realistically representing non-stop races.