Robin,
With the 3 different venturi sizes, the two heads and the timed crankcase pressure availability your 5224 engine is very adaptable. The "B" model 5224 engines with the round Aluminum venturi Inserts do not have accommodation for timed crankcase pressure.
The 5225 engine has 3 venturi inserts available, high compression is obtained by removing the head shim that comes from the factory. There is no provision for crankcase pressure. There is a boss in the back of the crankcase that can be drilled and tapped for a low pressure crankcase pressure tap. The 5225 does carry a steel thrust washer hidden in the nose of the front housing that makes it much more accepting of starters.
Pat