There are three versions of the Super Tiger 90

The oldest is the S90 It has a round sunken machined head and the front bearing housing is removable

The S90K has a squarish four bolt head but the bottom of the crankcase is made like the S 90 with a removable front bearing housing

The current G90 in both Italian and Chinese versions has thw Squarish head and a one piece crankcase. It have a bigger rear bearing than the other two.

I had some G90's and I ran a APC 14 X 6 prop and they turned 11,000 rpm.