This is not the kind of car to modify, no offence. If you want to keep it, try making it reliable and restoring it into near to original state.
If you really wish to modify, sell it and buy a Diahatsu Charade, 86 and up model. That would be within your budget. There's still some good aftermarket for these cars and mechanics are well versed in Charade engine and suspension swaps.
Be wary though, modifying takes patience, knowledge and deep pockets.