adb - How to mount system in rw mode if no custom recovery

When I changed my build.prop for spoofing another device model, I copied build.prop from another device having gingerbrad but my system is froyo. When I rebooted, I lost my internet connection. It reports no ruim card where as it is there. I had installed apks for terminal emulator, busybox etc. but they are not working. Xplore file manager does not work. But free version of Astro File Manager and ROM Toolkit work. I tried to push old build.prop on device using adb but it gives permission denied error as it is read only system. I came to know that adb root and adb remount works if you are in recovery mode. I tried to go into recovery mode, but there is no custom recovery installed on my cheap android phone, so can not go into recovery mode. su does not work in my adb shell. Can anybody tell me the way to mount system partition in rw mode using adb ( without recovery mode ) or is there any tool which will work in this situation ? Thanks in advance.