Assuming your estimations are correct.

Lets assume 45% winning chance of the top 2.
Generally I'll assume top pick wins 25% and second pick wins 20%.

If your 90% sure pick 1 can't win then it now has a 25%*0.10 chance which is 2.5% chance.

So your new chance is 55% chance for any other selection to win. 2.5% chance of your top pick winning = total chance of 57.5% chance of winning.

Your second pick now has a 100-57.5 = 42.5% chance of winning the race.

Of course these are all made up numbers as you can not be 90% sure your top pick will lose and if you did then you would look at excluding that as its no longer your top pick.