You'll have fun with that ,they are a bastard!
I fit the gudgeon pin circlip into the piston on the cam chain side,
Then use a normal piston ring compressor to fit the piston into the bore.
I find it slightly easier going in from the top as the taper at the bottom of the liner allows the compressor to jam.( that's just my preference)
Then I push the piston carefully down so I can insert the gudgeon pin in from the right side,it's a very close fit but the oil ring will stay compressed if you are carefull.
Then I lower the barrel/piston onto the con rod,push the gudgeon pin in and then fit the r/h circlip.
The circlips are a really tight fit and it's worth practicing with the old ones to get the knack of it so you don't over stress the new ones.
Make sure the crankcase mouth is packed with rags so the circlip wont get dropped .
Hope that all makes sense