Globulation 2 is a 10 years old Open Source cross platform real time strategy game that has grown to 50.000+ lines of code.
As it's binding only some developers that commit less and less code each month, something has to be done to boost development.
Tasks I would support would be:
** renaming stuff for consistency
** organizing classes hirarchically as now almost all is in one folder and in one case 40 classes are in one file
** removal of redundancy. quite some code/functionality can be found in more than one place
* resolving issues on the bugtracker
* implementing new features
* new graphics
* new sounds
* maybe even port the whole project to java
Some important issues are
* the project must stick to the rules of the gpl.
* the project must run cross platform.
* changes should not break history in the vcs (mercurial).
As a first shot I have no idea which of those tasks and at what scale is best to be cared of by a freelancer (a freelancer might need strong guidance by predefined goals and quality standards while the project up to now had only hobbyists that were interested in the success of the project not caring about profits) so I've just named the basic fields I could imagine finding a contractor. Specific goals and quality standards are yet to be defined with the contractor.
The budget I would be willing to spend on a monthly basis would be at max. 450$ as long as the project does not gain popularity among other people willing to spend money on it, too.