The IRF520 is one possible replacement, though its not a simple plug-n-play. You will need to make additional changes, many of which are already written up as mods. Here is a decent place to start
MOSFET Experiment Information , The ERF-2030 mentioned is still available, but typically the cost is much more then the IRF-520 which can be had for well under a buck each.
If you want/need further info more details will be needed.