XV920 1982 racer build

The XV starter system is a bit of a disaster, you need to find service bulletins for the 'upgrade' parts (basically the drag spring on gear is very weak)
The first one we sold at dealers was so bad I pulled cover off to find out what was wrong while general manager was calling Yamaha to get some info. Yamaha said, 'It's completely normal' (then sent out service bulletins and replacement parts)
It was a very popular motor in Europe for sidecar motocross (which tends to be a bit spectacular when things go wrong)
I don't know how much tuning information is available but I do remember riding one converted to 1100cc with a lot of 'fast bits' inside.
It was about as scary as a Suzuki RG500 and probably around the same power output (~115bhp)
May not sound like much today but in mid 1980's before suspension and frame technology had caught up with engine development it was fast

OH, your original question, the Allen Head bolts look like replacements with a taller head, possible wrong gasket fitted to cover as well, need OEM gasket to get correct clearances

You need to research the 'Doctor John' Moto Guzzi racers.
He off set the drive shaft to fit wider rubber.
Can you weld?
You may have to learn.and build a new swing arm, then you could go to a more common 180x17 rear tyre.
You CAN offset the rear wheel but it does cause problems cornering one way or another plus you have to counterbalance the offset. Many manufacturers have done it but usually only 1/4" or so.
Most common is Harley Davidson, but, you don't normally talk about good handling and Harley's in the same sentence.
I forget which model HD (early one, 1940's/50's) but you set it up with wheels pointing in opposite directions as the torque pulled things back in line

Yeah, I kinda forgot how much obscure nonsense I've found out over the years.
Had some great 'discussions' with the Harley instructors at MMI