If you were going to go with a four-cylinder engine, why would you choose the older U20 over the newer L16, L18, L20B or LZ22? The U20 was Nissan's first OHC engine, but the L-series was produced in far greater numbers and was considered a big improvement over the U-series.
You could even go with a newer Z22 or Z24 (80's Datsun pickup) and have much greater reliability, torque, and parts availability.
Not criticizing....just wondering what the appeal of the U20 is. That engine put out a stock 96 HP, or just about the same number as the 1.6 liter L16. The U20 could be built to be fast -- heck, the BRE team did it -- but the Roadster was much lighter than the 240Z and the L-series engines pushed the heavier 510-610-710 series cars very well.