
Tom, the smaller engines generally come with 54mm(?) clutch, and the 32cc or larger motors use 76mm clutch. the HONDA GX50 and larger engines may need a STATON adaptor to fit where 76mm clutches fit.
generally speaking, fitting engines in frame and rack-mounted locations is one thing. GEARING them to provide fair-to-maximum power is another.
Your 70cc engine is blessed with 4.1:1 gear reduction at its sprocket. that allows a smaller(30t-48t) driven sprocket. without this 4:1 reduction, the rear sprocket must be 123t-197t to attain that same gear ratio as the 30t-48t sprockets.
That is why the GRUBEE box is used, to provide gear reduction to facilitate the installation.
Myron