Hi,
well, as UCC replacement you might use Ubuntu with the Univention Join-Client. OPSI is still under devolpment and it will be released for UCS 4.3, yes. But I can not say when. Either be patient or look for another solution.
Afaik besides of OPSI there is m23 but I hav no experience in it.
Besides of UCS build-in app solutions you could set up a PXE-/Clonezilla-Server on your own. It is not very complicated, but definetly more error-prone compared to the build-in solutions, indeed.
Setting up a PXE/ TFTP server is partially described here. And for the Ubuntu-part take a look here.
Hope it helped.