I don't know if arms other than the Tabriz will drop in. It'd be nice to expand choices that don't require major surgery.
looking at the picture of the Radius 5, it appears to me that ANY Rega form tonearm will be drop in compatible. If you shop for arms, they normally identify if they are Rega, or Linn standard mount (these are the 2 primary standards that have been adopted it seems). In addition to Rega, Roksan, and Clearaudio arms, Bluenote, Michell, and Origin Live arms are all Rega standard as far as I know. This leaves you many choices. I have tried the O/L modified Rega RB300, Tabriz Zi, and Clearaudio Satisfy arms. I like both the Tabriz and Satisfy more than the RB300, though there are many Rega derivatives to consider so my comparison is hardly representative of all Regas.