Considering the smallest chainring for a 110mm BCD "compact" double is in the low 30s, the cheapest solution is probably that. There should be compact doubles falling from the trees on Ebay (not to mention in the ISO/trade subforum here), and getting the exact chainrings you want should not be a big problem (e.g. https://www.ebay.com/itm/Camapgnolo-...AAOSwdd9e-jXt) unless you're really stuck on Biopace.
In that case I suggest you purchase a complete Biopace triple and get it rigged for your bike. In my experience of converting from double to triple, the FD is a bigger issue than the spindle, i.e. the original spindle has worked just fine most of the time, but often I've needed to get an FD with wider range, because a triple crank typically moves the outer chainring further out than a double, at least in my experience.