If you are going to update, I would recommend a Raspberry Pi over the Arduino. More flexibility, fewer restrictions, and in the grand scheme of things, the same price.

Does your control look like this?
https://www.youtube.com/watch?v=BpyDre_IuVo

The machine is simple enough, I don't really see much benefit of my 6809 controller vs what I saw in that video. They just took two timers (mold closed time & mold open time), and a temperature control, and replaced them with a cpu.

The biggest benefit to going with a home-brew or PLC control is an easier implementation of the ejector time-out timer setup that I tacked on. In your case, you have a compelling reason to try...you might be able to get two shots to work. No guarantees, though. Unless you are doing it for entertainment, you are probably better off finding a larger machine.

What volumes are you trying to run? I have a few small machines that have much more capacity than the Rabit.