A new ring set is fairley expensive as they take an extra ring but the set is available from Hastings.  I bet you could use a cheaper set of the right size rings and just skip one of the compression rings with no problems...... it's likely better if you did as there's to many rings on them anyway.

 

I put a set of ones from a 4" stroke (WD-WC) engine in a Gleaner E engine (4.5 stroke) , trimmed the tops to pop out of the block and give a quench.  I cut the bowls out to relieve the compression down to a reasonable ratio and cut some valve reliefs in the tops.  Stock cam set at a 105* intake centerline and the engine sure lugs good for what it is.  Quench is important, keeping a piston down in the hole so far isn't the best solution, they give up some torque and are prone to detonation but it's usually all we have to work with in these engines.