Friction losses in a suction pipe will greatly reduce pump output. You need to compare the original suction line to the longer line, using a friction loss table. If the original suction line was, say, twenty feet long, and an inch-and-a-quarter in diameter, the newer hundred-and-twenty-foot suction line would probably need to be a two-inch diameter pipe, to give you equivalent performance. If you can switch the nozzles in the sprinkler heads, you might regain the original coverage. You'll have to run the system longer, but if you have the time, the lawn won't know the difference.
