I contact the developer by e-mail and was able to pay using paypal, after which I received a link to the .apk to download and manually install.
I sent the developer an email this afternoon, and he replied only a few hours later with a link! I paid (with VISA) and already have CalenGoo installed. So far, so good!
However, it could be a while before I'm able to make full use of it, as transferring my existing agenda to it turns out to be harder than expected…
I have 25 years' worth of data in Psion Agenda. I already found a Psion program called vCal which can convert that to a .vcs (vCalendar) file. It turns out that CalenGoo can import .ics but not .vcs. However, I found a Java program which can convert .vcs to .ics.
But checking the result in a text editor showed some deeper problems. It seems that vCal has lost a lot of data: all repeats, all anniversaries, all alarms, and all embedded objects! I only had 30-odd embedded objects, so I can do those manually, but the rest would take forever (and I'd never be sure I got everything).
So I'm now wondering how feasible it might be to write my own conversion program…
(Edit: this may be feasible in OPL, as Psion released several OPXs (OPL eXtension modules), one of which gives programmatic access to the Agenda engine. In fact, I already have it installed, along with its readme. If it gives access to everything relevant, which at first glance it seems to, then this should be doable. Also, .ics is a text-based format and looks straightforward, though it'd be useful to see some docs on it.)