QSECOFR provides a repo for IBM i that has Mono and the dependencies that it requires, as well as additional software for IBM i.
To install, add this Repo file to
/QOpenSys/etc/yum/repos.d. The packages are built for IBM i 7.2, and should function on IBM i 7.3 and IBM i 7.4. (AIX is not supported.)
All software has SRPMs; the patches and specfiles they are built from are on GitHub.
This is not a complete list.
As well as any dependencies.