there is a program called "Screenmode AmiKit" in AmiKit:Prefs folder. The purpose of the program is to adjust all the icons and windows according to the screen mode you select. Unfortunately I cannot support all the screen modes because it would require lot of presets for many many screen modes.
However, you can select similar resolution in "Screenmode AmiKit" program and then use the regular AmiKit:Prefs/Screenmode program to set the exact screen mode according to your needs. This way the icons and windows will be adjusted approximately at least.
As far as I remember, "Screenmode AmiKit" supports 1280x800 resolution, which is the closest to you. Try it. Simply run the program, select desired screen mode and before you reboot, choose an option to select the same or similar resolution in regular screen mode preferences.
If you select different resolution in the regular screen mode preferences, note that the boot screen is still set to the mode selected in the "Screenmode AmiKit" program. In this case better disable the boot picture:
Edit AmiKit:S/Startup-Sequence and insert semicolon in front of the following line: