Question Can I force a 1366x768 monitor up to 1600x900?

You may be able to run it at that resolution if you select a custom resolution but it will just scale it to fit within the existing pixels. You can't add more pixels to the screen, that is fixed. You may also get an out of signal error if you try to do this and it fails but it should revert back after a few seconds.

For computers the best resolution for a standard system is 1080, it looks good and has few scaling or compatibility issues. Meaning replace the old monitor. You can find used 1080 screens for $30-40 for basic 22" models.