Just a quick update on what has happened so far. I managed to disconnect all remaining wires and tubing from the engine and also after some quite considerable difficulty was able to cut the bolts retaining the exhaust pipe using an angle grinder. The nuts and bolts were so melted together by rust that they could not be separated by conventional methods. Radiator and it's fan were also taken out.
After that I hoisted the engine a little with a cherry picker I was able to loan, supported the transmission from below and removed all the bolts keeping the two together. I then realised the drive shaft was still connected to the engine block from below. I got it somewhat separated, but was unable to remove the engine completely. At this point I ran out of both time and energy, put a few bolts back to keep the engine in place and decided to continue next week.
It seems that there isn't much space to move the engine sideways and I'm a little sceptical whether the clearance allows me to move it enough to make room for the flywheel come out. I'm still a little hesitant to remove the transmission though so I'll give it another go.