Hi.
The G20/19 (like many other S.T.) has undergone many changes over the years. The first edition had the head made of casting, the piston had a baffle and two rings, the shaft was supported by a rear ball bearing and a front plane bushing. The various modifications resulted in an engine with a machined head, a flat piston without a ring and two ball bearings. I have no information regarding S.P.I. but I would not rule it out especially in the first versions, the big brother G20/23 originally had two small open corners at the base of the exhaust port with the piston at the TDC.
Massimo