Stamped on, or cast into, the alloy body is the following:
Fiat
E76-0.5/12S - VAR.9
4112049
S
The S is for "Sinistro" indicating counter-clockwise rotation. The
pinion gear has 9 teeth.
A Bosch unit from either an air-cooled BMW motorcycle or a Moto-Guzzi
may well fit - count the number of teeth on the pinion gear, and note the
direction of rotation. Aircooled BMWs between 1970 and @1998 have either
8 or 9 teeth on the pinion gear. If one of these fits, then an alternative
will be the French Valeo unit from later model BMWs including possibly
the R259 oilcooled twins. The Valeo is much lighter, but not as reliable
as the Bosch.
The diagram below illustrates a unit similar to that fitted to many
G109 motorgliders.
If the starter motor turns the propellor slowly, and yet the battery
appears to have both good charge and cold-start performance, new brushes
may be the solution.
Moto Guzzi starter motor Valeo replacement:
"Just transplanted a Holden Barina motor onto my Cali3. The motor came
from the Spanish built car with an Opel engine."
http://www.mypage.tsn.cc/rdd/TT/contents/ELEC/starter.html
Battery
Odyssey Battery in G109
PC680MJ
7.27 (185) x 6.67 (169) x 14.7 (6.0)