A newer style yoke can be fitted and many Scubapro dealers who've been around awhile will possibly have a shiny chrome heavier yoke available.
I'm not sure when Scubapro switched to the stepped piston in the Mk 5, but Scubapro has created some liability issues for some shops/techs by recommending the earlier style pistons no longer be used. Should one of the older straight stemmed pistons fail or cookie cutter the seat, the liability for the tech and shop is now pretty large as they are violating Scubapro's service clinic recommendations by putting one back in service. Since replacement pistons are no longer available, some shops will be hesitant to return a Mk 5 to service with the early style straight stemmed piston.
The good news is that Scubapro still makes the Mk 5 annual service kit.
And, IMHO, since Scubapro moved away from the SPEC system, the older Mk 5s with the larger ambient pressure holes in the swivel cap are probably preferable as they are easier to rinse than the last Mk 5's that had the small ambient holes for the SPEC system.