It is not so simple.  If you replace an Iskra after many hours of operation with an S-tek, the S-tek is likely going to start slipping due to the cumulative wear from the Iskra.  If you have had an Iskra for more than 100 or so hours, you should probably leave it alone.

Either the S-tek 3 or 5 will work.  The 5 is recommended for the TSIO-550.  I have a 3 on my TSIO-550, and it works fine and is lighter than the 5.  Note that the 3 is a very noisy starter (sounds like a coffee grater with ball bearings in it), but it works great.

Pete