The amazing news is that on Monday 25 September 2000 he got a letter back from him, including an autographed caricature of him as Pigsy!
Here's the autograph (click on it to see a larger version of it):
In the letter, he mentioned that he was very happy to receive his letter, plus he said the following:
"Monkey" is, as you know, based on a famous Chinese folk tale which is very popular in Japan.
The program was made some while ago. It was actually made 22 years ago, I believe.
To me, Monkey is one of the most memorable TV program I've ever worked for.
Although playing a PIG was not easy for a HUMAN actor, the program became a huge hit in Japan and it gave me a big break-through as an actor. They were tough shoots.
But I'll never forget how I enjoyed playing the part and I'm really glad that you enjoyed watching the first episode. I hope you get a chance to watch the rest of the episodes
If you'd like to write to Toshiyuki Nishida yourself, you can write to him at the address given on the Monkey Cast - Toshiyuki Nishida page.