Riquelme, if he's free, would be brilliant. He's very good on various counts but be aware that his teamwork and work rate are pretty low. So if you like your AMC's to track back at times and help out in defence etc, you can forget it with Riquelme. But yeah, he's a brilliant player. And at 29 will give you atleast 3 solid years.
Crespo, I'm not so sure of though. Yeah he's still decent at 33. I suppose he'll be ok as a backup but is he willing to play that role? Also, do you really want to pay his high wages if hes just a backup?? He cant play instead of Aguero or Forlan. He's not that good anymore.
If you do well in the league and the UEFA cup and the others, you'll probably have a fair bit of money coming your way. So your transfer budget for next season should be decent no? (I really dont know how AM's transfer budget is because never tried to play as them!). Dont sign a player because he's free. Its much better if you spend some money and get in another class striker. I think every club needs atleast 3 world-class strikers. Aguero is one, Forlan is really good as well and you can do with one more I suppose. So instead of Crespo, try to sign someone. Mista would then be a very good backup.
What happened to your players like Simao, Maxi, Reyes etc?? I would have thought they should be playing more?? All three of them are very good. I suppose you might have sold them eh? If not, get rid of the one you use the least. You should get a good amount for him. Use that to buy a striker if you dont have any transfer kitty left.