The Twin Cam has an inch longer wheel base than the S1-S2, most of it in the engine bay. The S1-S2 chassis forks at the rear, making one long, narrow-angle 'V' back to the chassis cross-member.
The Twin Cam engines timing chest that encloses the run of chain to the overhead cams is much wider than the front of the pushrod Renault engine. In order to fit around the wider engine, the Twin Cam chassis' legs fork at a much wider angle, reaching full width much quicker, then running parallel back to the chassis cross-member.
The wider Twin Cam won't fit into the narrow S1-S2 chassis without notching the chassis, then reinforcing the chassis to compensate for weakening it with the notches.
It can be done, and dedicated owners are putting Toyota 4AGE and Ford Zetec engines into S1-S2 Europas. It's not a simple bolt in, and requires significant chassis and suspension modifications.