It took a couple of hours. I started with a midi file from MuseScore.com This had about 12 parts, using different recorders (tenor bass etc). I copy and pasted this down to 3, then I had to put dummy notes (used a high note not in the part) into all the complete bar rests, as my conversion script cant handle say a section of 6 bars of rests. I also adjusted tempo changes to get suitable rit at the end. (For SP you need each part to change tempo at the same time, unlike in midi). I then separated the individual parts and saved them as musicxml, and used a processing script to convert them to SP format. I then copied the converted parts to a text editor (I used visual studio code) replaced the high notes with rest symbols, added synth and fx info, and played with sustain release settings to get final code.
It sounds long winded but its not too bad, and a huge amount quicker than entering the notes and durations manually.
I did it with 12 parts to start with which lengthened the time taken. Then I thought as 10 of the parts don’t overlap I could combine them. This reduced the number of dummy notes that had to be added, and made the whole process a lot quicker.