Sum-up from the comments:
First, at least as of today and outside the EU, it is not possible to install a custom ROM or even only root your phone without voiding your warranty. Even if it can be done in a way not increasing a (possibly existing) "flash-counter": In case your device "bricks" for some reason, not giving you a chance to restore to "stock", your custumizations would be found in the service center.
As said, here you might want to take a look into the question Where can I find stock or custom ROMs for my Android device?, which lists a lot of ressources where to find different Custom ROMs. A good choice might be CyanogenMod, which is e.g. dealt with in the XDA-Developers thread about CyanogenMod for Ace Plus. To find more details, you can simply use this Google Search.