First pic, No5 bayonet blade top, L1A1 bayonet blade bottom. The fullers look to be visually almost exactly the same length but I have not measured them with verniers or anything. The L1A1 blade tang in the example shown, a new replacement blade blank, is a little larger than that of the No5's tang. The No5 bayonet blade shown may have been shortened slightly (blade tip) in use/service?
2nd pic No5 bayonet blade on top of L1A1 bayonet blade.
I do not wish to part with the No5 bayonet blade shown as it is part of my No5 style blade collection. However I may have been able to find another new, unused, L1A1 blade blank, similar to the one shown in the photo. I would have thought that someone with your knowledge, experience and skills would be able to grind the tang to the size/profile of the No5 bayonet and drill the appropriate holes in the correct place. If you have plenty of offers of No5 bayonets at very reasonable prices this, of-course, would be a complete waste of time.