Stick with Ant since you're building Java. I've mixed Ant/SCons for some JNI work, but in general I'd stay with Ant especially since you've got an existing build setup in Ant. Porting to Maven will be like shoving a square peg through a wall with no holes.
Embrace your custom Java logic for any and consider writing proper Ant tasks instead of executing external Java code. I solved some very complex parts of our build process by simply extending Ant to do exactly what I needed, ex. managing icon resources for a large gui project or injected subversion information directly info jar manifests (thank you SVNKit)
