Lets go down the list:
- Is the room clean?
- Are you doing the surgery on a bed?
- How well lit is the room?
- What type of medicine are you using using?
- What is the skill level of your doctor?
- Are they suffering any manipulation penalities from drugs or injury?
I dont know of a mod to rig surgery chance but this is the list I run through when Im having surgery issues. Also, you can just save scum.