Probably not(25" is right), but you measure barrels by running a cleaning rod into a closed bolt rifle, marking the rod and measuring.
Looks unaltered. The ramp is a sheet metal thing used on commercial sporters.
Given that the rifle has been drilled and tapped, it isn't the best candidate for restoring to military configuration.
Value. Wouldn't have any effect on replacing the missing/altered bits.
Personally, I wouldn't restore this rifle, unless there was something pretty special about it. There are lots of sported SMLEs around, which don't have extra holes.
Restoring a SMLE is more complicated and expensive than restoring a No. 4. More parts, which are harder to find, and more expensive.
Seeing as it is already drilled and tapped, scope it and set it up as a nice sporter. A sporting style buttstock makes all the difference in the world.
Or, since it's d&t'd you could restore it, mount a nice looking vintage scope on it and have a guilt free scoped SMLE. I'd do it.