After trying several combinations on my workbench, I have confirmed that an unmodified Mauser 98 triggerguard will mate successfully with a Springfield 1903 receiver (which is kind of a Redneck wedding, since the 1903 is actually a Mauser anyway). Depending on the parts you are using, the only issues may be:
1) wood inletting around the trigger guard may need minor modification;
2) Springfield action screws have a slightly larger head than a Mauser, so may need turned down a bit to fit the triggerguard properly;
3) the hole in the stock for the rear action screw may need drilled out slightly to allow the screw to align with the rear tang correctly.
I forgot to mention that hinged triggerguards from standard length military Mausers will also fit, including the Argentine Model 1909 and Portugese Model 1904 Vergueiro. The Spanish Model 1895 also had a hinged triggerguard, but is a shorter (intermediate) action and won't line up correctly with a Springfield 1903.
Years ago there were lots of new aftermarket parts available for sporterizing Springfield rifles, which had become a cottage industry from the 1930s through the late 1960s. However, those days are long gone and quality 1903 sporter parts have become very hard to find.
Sunny Hill Enterprises is/has made a run of commercial hinged floorplate TG's for the 03 Springfield. I would guess they would be in the 400. range as that is the Mauser Tg price. They do make a nice product.