Have it welded by someone good. If you don’t know someone good go to your local airport and talk to the A&P mechanic. Chances are he will know someone.
You can save yourself some money by doing the pre cleaning yourself. Don’t take the welder an oily mess. He won’t be pleased.
Throw the bike in a truck and take it by the recommended welding shop for a consult before you disassemble it or do any deep cleaning. Find out what the guy welding it thinks.
The guy doing the job will tell you what he needs to have done before it shows up to be welded. They will also be able to ball park the cost.
At that point you will have a good idea if you should go that route or take a different path.
You want the best welder you can find to do that job because of the level of contamination and that is a casting.
If the motor doesn’t need to come out, do pull the ecm, Speedo if it’s not strictly mechanical, regulator, and anything with an LED in it at minimum. AC and high frequency start can create stray currents that will wipe out or weaken sensitive electronics no matter how close you attach the ground to the work.
Thats been my experience anyway.