Probably something that happens on all DTs and most other bikes.
On my 400C the kickstart and the part of the kickstart that goes onto the shaft had worn over the years, which meant that the kickstart pedal was no longer sticking out parallel to the kickstart shaft, which meant that the pedal was pointing slightly rearwards - just enough to make the pedal point slightly rearwards which could make your foot slip off.
My solution was to drill a small hole - 3.5mm in the kickstarter pedal and then make up a steel pad 8mm in diameter and with a 3.45mm spigot ot that fits into the hole in the kickstart pedal. The top of the pad was then filed so that its profile more or less matched the shape of the part of the kickstart that fits on the shaft.
After much filing the kickstart pedal now sticks out as it shoul do. I used silver steel to make the pad which I then hardened after I had finished profiling it.
How long it will last is anyone guess, suppose I could have built up both parts with weld, but as they are cast items they are difficult to weld.
Something to do on a grey / cold / wet UK day.

Snap a pix to show what

ckiff

In my head I used the word "arm" or "lever" instead of "shaft"... because I think of the "shaft" as the splined part that sticks out of the case that the "arm" fits onto. The pedal is the part on the arm that swings out to kick and then folds back in out of the way.

I've never really seen one wear on any of my Yamahas... but I have seen a few aftermarket kick starters where the pedal fit to the arm is sloppy and folds out too far to the rear.

Thinking about it, the 400 could have the type of starter where most of the arm pivots on a pin very near the splined shaft...  

I'll see what I can do in terms of a pic or two - watch this space.......

A picture - you will be able to see the 8mm dia part I have added to the lever.
Both parts have some wear, the 8mm part fills out the worn areas.

  

I just build the stop area up of the lever or the boss part with arc welding rod called [Hi Ten 7] by memory. Good rod for welding metals like what kickstart is made from & always have a few rods on hand. I guess there's better ways of welding these days with modern gear, like Tig welding.

You are right welding is the way to go.
I have a small MIG welder but would probably need some specialist welding wire and or gas.
I'll see how long / well my repair lasts - I do have a specialist welding shop a few miles away so could get them to build it up so I could file / grind back to size.

I just had another look & expanded your pic up & now see what you've done & trick too!!.

Nice job!