I am a BIG fan of .22 LR for all kinds of things, but I hate .22 LR conversions.
I would highly recommend an M&P 15/22 as a base for an SPR surrogate. You can use your preferred stock, trigger, grip, and optic, and very accurately replicate the aspects of the rifle that really matter.
The advantages of using .22 for training are well established, but I like for the following reasons:
.22LR drop and drift both scale pretty well in comparison to centerfire, so you can replicate longer distances with appropriately scaled targets
Reduction in distance needed for high value training repetitions makes finding a training location much easier
Reduction in projectile energy makes reactive targets last much longer, and can be made of thinner/lighter steel
Low sound signature reduces the need for suppressors (though 22 suppressors are exceedingly awesome)
Low recoil helps focus on foundational skills
Low velocity (increased projectile time in bore) reinforces good follow-though and trigger interaction
Significantly reduced cost per round, and shipping cost
22 LR barrels basically last forever if you make any effort to maintain them at all
Increased wind effect on trajectory provides immediate motivation to become very good at wind reading