Tire size is going to depend on the clearance you have side to side as well as top to bottom. Your current tire is 130mm wide with a sidewall height of 117mm. (130 X 90% of the width) You can do the math on other sizes to see what the size difference is.
I have checked a few tire size options out. Running a shaft drive it looks like I have more room to go wider. The math seems to work, but I always value someone that has tried it already to see what they have to say.
Back in the day it was common to go up one tire size, but changing the back only is going to make it seem like the bike is falling into turns and reaching around from the rear to catch up with the arc of the turn. I went up one size front and rear on my GS1000E this time and it handles like it is on rails. There should be plenty of room to do the same on your bike. I believe the Shinko 230 Tourmaster series will have the sizes you want. They handle very well and are reasonable in price. 110/90-19 front 130/90-17 rear is one size up on both on my bike.