There is nothing you can do about it, if you don't have the original application source code, I'm afraid.
If they already are Series 60/Nokia UI (User Interface) API (Application Programming Interface) based apps using the original Series 60 176x208 resolution, then they should automatically scale to the larger resolution display of the N90 (i.e., every pixel is automatically multiplied to four).
As they don't, based on your description, those games have most likely been developed with "generic" Java (MIDP) APIs and cannot/do not use the whole display.
If you can't live without those games contact the original software developers and ask for "Series 60" versions, if they have them (or at least those using Nokia UI, User Interface APIs).
To really take advantage of the higher resolution display, the apps must be rewritten/recompiled for it, using "Series 60 2nd Edition, Feature Pack 3" (Series 60 2.8) compatible tools (from http://forum.nokia.com/).