It's the biggest kit for Minarelli Honrizontal engines using the stock stroke 39.2 and 80mm rod.
It's a 77cc 50mm.
Here's some specs I copied from a Belgium site if that helps.
Cooling: H2O (liquid)
Power: 13.800 rpm 26 hp
Power couple: rpm 13,500 13.5 kgm
Number of transfers: 7 T + exhaust has bar
Material: Aluminum nykasil
Scheduled to run: 39,2mm (original)
Scheduled for rod: 80mm (origin)
Recommended Squish: 0,50mm
Advance has advised the ignition: 16 °
Recommended compression ratio: 1: 15.50
Recommended ignition: malossi mhr sellettra yellow
Recommended Carburetor: Dellorto 28mm VHST
Recommended plug: W34ES-ZU
Exhaust diagrams: 196 °
Admission diagrams: 130.5 °
Capacity of the combustion chamber 5.3 cm3