As far as i know from the last 15 years flying BO 105 there is no certification from FAA or a single european country including the great JAA
to fly a BO 105 SPIFR. In the manufacturer RFM including supplements, it needs every time a 2 pilot crew to fly on IFR conditions and a working SAS. In my understandings of the international air laws that should limit the operation to DPIFR and it makes no difference which qualification the single pilot holds or if the helicopter is in controlled/uncontrolled airspace or the kind of opinion the local authority have about IFR flying. If you ride single on IFR you operate the ship outside the manufacturers and certification authorities green area und you will lost your insurance cover.
Alternatively, can they be made single pilot with newer avionics?
Eurocopter will not support further improvements of BO 105 because the a/c is long out of production.