This may be impossible in all the cases where the original authors did not provide sources of their plugins, or did not document their algorithms detailed enough to reprogram them. But where it is possible, a lot of efforts were already invested to implement them. The AviSynth Wiki lists
many successful attempts. If you know some missing ones, always provide links to their original location, optimally with sources. A few special plugins may even be easily resembled using very generic tools like convolution or LUT functions.
Specifically regarding "RedAverage",
Average is reported to do the same, just faster.