What year was Bertha birthed? There are differences in early v later R65 models.
Adjust your idle rpm to be at least 1000 rpm. If the engine idles too low, the GEN lamp reminds you that the battery is no longer being charged. Also, the design of the timing chain and sprockets is such that at a too low idle, very little or no oil is reaching the chain, sprockets and forward cam bearing. It's a rather crude design that simply spits oil toward the chain. Low revs will drop the oil pressure and the timing chest components wear for want of the crude splash lubrication design.
The Type 247 engine likes revs and is happiest from 3K and up. Avoid lugging the engine in higher gears.
Set valves prior to compression checks. Also, the carbs should be removed from the heads as the CV Bing's vacuum operated butterfly valves will not be open. Free air flow is needed. False compression readings result. Check the six head bolts for correct torque but do not exceed 25-27 pounds or there is a danger of stripping the aluminum threads in the crankcase. See your manual for head bolt torque sequence.
Adding a second disc will require sourcing a fork lower with the correct mounting lugs. PWhat brake caliper do you have? ATE or Brembo? Properly adjusted, the single disc will provide excellent braking. Duals will show a slight improvement but add substantial weight to the front forks and may require add'l tuning of the suspension innards. Twin discs do look sporty.
A 120 rear tire will be difficult to fit between the swingarm and may likely rub. Depends on the model year and the brand of tire. Bridgestone S11 Spitfires are popular and well match the modest suspension performance.
Post some photos. You will need to utilize a photo hosting site to post on the forum. Photobucket is a good, free service.