Irelander: there is a bit of confusion here. You said the rifle was a 1917 Enfield in 303 British caliber but the markings under the bolt indicate it is a No3 Mk1 Short Magazine Lee Enfield (SMLE) made by Birmingham Small Arms in 1916. The later rifle, the SMLE, can not be safely converted to the 308 Nato cartridge. However, ya really ain't missin' anything with keeping it in 303..........
I would give the barrel possibly the best scrubbing it has ever had and then take a real good look at the enitre length of the barrel. The rifling in modern rifles looks pretty thin and for an old warhorse there may be some pitting but this may not affect the accuracy of the rifle much. Clean her up and shoot it and see how she does - she may be fine for you as she is. HTH. Mikey.