RE: Quique Somenzini YAK-54S 73
Place the engine so that the cylinder head points at about 130deg (8:00hrs clockwise as seen on the firewal)l and u can use the stock OS muffler. This is a common practice recommendend by Great Planes at their manuals.
No problems with engine - it runs as reliable as ever.
YS is by all means a very good choice. Consider a soft mount if U go that route (weight). YS are powerful beasts that produce increased vibration at both tickover and throtling.
Still the best choice if UR familiar with these motors (I only had the 110S for some time). LIghtweight and powerful but tamperamental IMO.
P.S. Get yourshelf a good tank designed for high pressure systems to get a reliable running YS -possibly by TETTRA from Central Hobbies.
Good luck.