The original Geminis that I worked on had the Cirrus minor engines.
We had several at Burnaston and we modified two of these from the Cirrus to the Gypsy Majors.
These were Percy Blamires and I think Fred Dunkerly's.
I worked on both conversions ( as mere apprenti!)
The main thing was that new engine bearers were installed
which was quite a major rework.
The side panels of each engine were rivetted to the bearers and these used hard rivets that had to be softened to drive them. To this end in one of the back sheds ( well away from habitation!) we had a salt bath to heat treat these rivets and as apprenti my job was to monitor this treatment. The bath as the name implies is full of salt which is then heated to molten ( quite hot!!) by gas jets.
The room where this bath was located had all safety poster stuck up showing the dire effects of a drop of water falling in the bath.
I took several hours to get the proceess complete all spent looking at pictures of various demolished buildings
The said rivets were then put in a vacuum flask to keep them workable for maximum time.
I flew in the aircraft several times with Percy who had an artifical leg, the foot of which was firmly strapped to the rudder pedal!
I always thought that this conversion made it an Aries but that may not be technically true as I think the later production aircraft had Gypsys from new.