Get a PC. The GBA has been emulated on what were then fairly modest machines since day 1 and that was the better part of 19 years ago, there are also several well developed debugging capable emulators for it that make this sort of thing considerably easier.
Anyway if that emulator or another on android does not do cheat finding (I tend not to find such things on android for whatever reason) then I guess you could attach a memory viewer or dump the memory of the program. From there figure out where the GBA's emulated memory is stored in that dump and proceed to make cheats as per usual. A lot of people use techniques like this on the PC (stuff like emuhaste, artmoney, cheat engine).