Can I remove my M1a hinged butt plate, replace it with a non hinged one and shoot in the NRA Springfield M1a matches? I am new to my M1a and really dislike the hinged butt plate.

Looking for a suitable plastic or polymer alternative.

You have to use the issue buttplate to qualify for "service rifle" rules.

I believe you can use M1 Garand buttplates in lieu of the M14 style because a limited number of M14's were issued with them.

Ugghh, well that just sucks. Is there a butt plate before the hinged one came into being?

Yes, the original M14/T44E4 stock mentioned above.
You could use a filler in a wood stock to replicate the dimensions.

Probably because it's slip and slide on his shooting jacket during rapids and the only remedy is spray-on sticky gunk.

All great points + it's too heavy for what it is.

If you can't find or don't want to spend the $3-500+ a T44 stock sells for, you could find a GI spacer P/N 7790914 that allowed for the mounting of a Garand buttplate on an M14 stock circa 1960 when there was an M14 buttplate shortage. Originals are impossible to find, I believe some repros were made. A passable imitation can be made with a hunk of SAI rubber buttpad.

I have fitted a Garand buttplate to a wooden M14 stock. The hole spacing is the same between the two. I cut a wooden block and epoxied it into place to fill in the area where the M14 buttplate hinge normally sits. This was not an easy thing to do.

Perhaps I'm missing something, but the relevant section (3.1 "Service Rifle") of the 2020 edition of the NRA High Power Rifle Rulebook says (emphasis added):

"(a) U.S. Rifle, Caliber .30 M1, or caliber 7.62mm M1––A device consisting of modified cartridge clip that is intended to permit single
loading from the clip into the chamber during slow fire is considered
an internal alteration to improve functioning and is permissible under
this rule.
(b) U.S. Rifle, Caliber 7.62 mm M-14––Must be no more than 2 inches
wide at a point immediately to the rear of the front band, no more
than 2 1/2 inches wide at the front and rear of the receiver, and have
a continuous taper from receiver to front band. Width at receiver
may be carried through to the butt plate, which may be the flat M-1
or hinged M-14 plate, which will be used only in the folded position. In all courses and in all positions the standard 10- or 20-round
box magazine or a reduced capacity magazine of the same external
dimensions will be allowed. For stock dimensions see diagram."

Based on this, it seems OP is GTG.

Yeah he's GTG per the match rules, but he doesn't like the "shoulder thing that goes up".

Well, it is pretty useless in a match environment, being intended for fully semiautomatic spray fire from the hip with 30 clips of thrust per squeeze.

Agreed.

I've actually used it for its intended purpose in prone, and the hinged shoulder rest is useful in that role.

Apparently I don't have a video of that, so here's a mag dump where it isn't used. lol