Timbo,
You firing pin worked fine with original hammer correct? So it's not the firing pin.
When changing hammers in New model Rugers, the first thing to check is the transfer bar clearance with hammer as JLarsson posted.
TO TEST:
With hammer fully cocked, press on the transfer bar to extend the firing pin and observe how far thru the recoil shield it protrudes. Now drop the hammer, keep the trigger pulled back and observe the firing pin again. If it doesn't protrude the same amount, you need to remove just enough metal from the hammer nose so the transfer bar is pushed tight against the firing pin and hammer channel. If you remove too much, the transfer bar will not perform it's safety function. If firing pin protrudes the same amount, the problem is too weak of a spring. Also make sure hammer has no friction or contact with grip frame ears around it's base to slow its fall; another potential issue when making hammer changes.
IF YOU MODIFY HAMMER NOSE:
At the range, double check the safety function by loading a live round under the hammer, point down range, cock the hammer, hold with thumb, release the trigger to uncock the hammer, take finger off trigger and release your thumb so the hammer falls. The round will not fire if safety transfer bar is functioning correctly.