The 350 stoker, along with the 700, were discontinued about 30 years ago. The 900 is still manufactured by Anczarski in Shenandoah. efm has been producing the current 520 for the last 56 years.
Some of or many of the S15 stoker parts will fit on the S20. Using the old BTU output charts, the 350 had a gross output of 112K BTU and a net of 84K BTU, compared to the 520's 166/124K output.
If you look at the online manual at http://www.efmheating.com
for the DF520, you will be able to find out alot of information about running your stoker.