Is there an easy fix to reduce weight of pull on a commercial Mauser single stage trigger such as is on the early FN's? Will a lighter weight spring work? Is it safe and where could one obtain a replacement spring? Thanks.

The spring isn't going to do it.
And modifying it really wont work either. The pull can be smoothed out by polishing the sear surfaces but that takes some precise care.
Buy a Timmny and be done with it. little over $50

I have lightened many by stoneing as kcstott suggests and have had good success with removing small snips of the original spring until desired weight of pull is accomplised. This should be done carefully and slowly to insure strong return of sear. The finest trigger that I own is an original mauser converted to single stage (front hump removed) with set screw through front spring (for over travel) and set screw in front of trigger (for amount of sear engagement).

Very decent triggers can be developed without actually lowering the pull weight. Both a trigger which has the pull smoothed by polishing, and/or a trigger with an inexpensive trigger shoe can FEEL significantly lighter. Sometimes that will be all one needs.

Depending on the original weight of pull, it is sometimes also all that one should have for use in the field. A really light pull can often be dangerous AND can substantially degrade shooter accuracy in the stress (excitement) of hunting.

Ablerta
Thats true as the trigger pull on my AR15 is a heavy 7 pounds it is crisp and did fool me into thinking is was much lighter.

My Walther Model B's have pretty nice triggers that they reworked and added a spring not unlike that of a New Model Blackhawk's trigger return spring. A tranverse coiled spring with legs and a wrap around the front IIRC. I am far from home so cannot go check them.

I have use Timney triggers on a couple two or three rifles and they are good but if I were doing a Mauser I would get a Bold trigger. The Bold is a better trigger than the Timney, in my opinion, and they aren't as pricey.

I'm just the opposite. I have, and use both the Bold, and the Tinmey, I think the Timney is a little smoother than the Bold, and the last two Timney's I've ordered had a wider trigger face and feels much better. For me it all boils down to what is in the budget at the time. I did order a Dayton Traister trigger once, what a POS.