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The square-tube engine mount/crossbrace is similar to the old PBS Engineering unit. A long time ago they made a kit to fit the Fiat 124 pushrod/OHV engine into the 850 chassis - this may be what is in this car. PBS also made an alloy transmission bellhousing (adapter) similar to the Abarth 1300/124 bellhousing.
The rear panel on this car is like on the OT2000 "Mostra" (2 Mila) Sedan, with the large bulge in the center. The engine has a dual-carb manifold like on the Abarth 1300 Scorpione.
This car does have what look to be a few original OT1600 parts on it, ie: front grille, dashboard gauge unit, steering wheel & front seats. I can't see the wheels clearly enough to tell if they're original Campagnolo Abarth or repro's(?). I'd be eager to see what the front/rear suspension on this car looks like...might give a bit more insight.