Delaying implementation of tolls on the Sakonnet River Bridge could kill the plan permanently, causing a loss of expected revenue officials call critical for bridge maintenance, according to Rhode Island’s transportation chief. The General Assembly pushed back the introduction of tolls on the newly built span — which were to go into effect in July — until at least February while a special commission studies alternative ways to fund bridge upkeep. But Transportation Department director Michael Lewis said it may be now or never. For federally funded bridge projects, tolls may be introduced only if they begin before the span is deemed ‘‘substantially complete.’’ That could happen before February, he said. The Rhode Island Turnpike and Bridge Authority approved tolls ranging from 75 cents for those with in-state E-ZPass transponders to $5.25 for those without transponders.