The only thing, and I mean the only thing I prefer about 2 part dosing is the ease at which you can adjust your levels. Fine tuning my knop reactor took more time, and was usually more of a reactive adjustment instead of proactive.

In answer to your original question, NextReef has a 2 part reactor that you could convert to a calcium reactor. But, after you add the circulation pump, fittings, media, and bubbler counter, you're probably approaching the cost of just outright buying one.

Keep an eye open in the For Sale section. I know a lot of people like myself are retiring their calcium reactors, so you may be able to find a good deal.