I think that 1st/4th (front) shift fork was too far rearward for some reason, and rubbed 4th, which gave you the shuddering in 4th.
It might also have been in the way of the middle fork moving 3rd/4th input forward when trying to engage 5th, making it hard to get into or stay in 5th.
What's the front end selector drum groove/ track like? Also the tit on the fork that sits into the track?
Real clunky and not positive at all gear changes. Is it cos the engine is the wrong way up? The selector forks don't slide in the selector drum very well. In fact one looks quite worn.
Is it just the camera angle or my eyesite but does that hole at the bottom of the shifter fork look oval?? If so, would that cause the fork to wobble on the shaft which would prevent complete engagement of the gears/dogs and the wear seen on the fork portion. What does that shaft for the selector forks look like? Can you wobble the forks on it? (PS, I've never heard of anyone having a problem in this area before???)