I want to refurbish an old McCall pizza retarder originally designed for R12. I work on more than a few of these old units. Theses units are constantly under heavy usage and the existing refer units just barely keep up in summer. Pizza Retarders are simply large upright reach in coolers, tall & wide enough for a large cart holding pizza doughs.

These units need be between 34F-39F. The door is opened often and the unit is usually near the ovens.
The units have all been converted to R409A, a few to R414A hotshot using Tecumseh 1/3hp skids, AEA4440AXA R12 compressors and Sporlan 1/4 ton R12/R134 TXV's. Some have been converted to a thermostat. Original equipment I believe used pressure control for temp control. I prefer the pressure control myself. I can dial these in between 16-34psi with R409 for suitable temp control. The TXV's are original equipment. Nobody has changed, no reason to. The evaporator sits about waist high on the inside wall, the condensing unit on top or the unit.

Moving forward, I want to beef up these units for no nonsense service in a hot kitchen with normal PM maintenance, In other words, I want to avoid the above 40F calls. The 1/3hp compressors work nice when new but grow weak with 2yrs or so and suffer in summer.

Can I move toward an 1/2hp skid? If I move to a 1/2hp skid, will this cause issues with an undersized evaporator? What if I convert to R134 along with a 1/2hp. The charts show slightly higher btu's with the R134 vs R12 skid in the UR catalog. Can 404A be considered with the old McCall retarder with the correct TXV? I don't understand how evaporator sizing comes into play with the different compressors & refrigerants.