Use a higher gear for bump starting, rather than first. You just need the motor to spin, it doesn't have to spin fast. When it catches just be in the seat and/or have your hand on the clutch lever to pull in immediately.
I can bump start my 700SC by just straddling the bike from the seat, walk it forward a few feet to build some momentum, and pop the clutch. Varoom.
You really need to track down the cause of that 25 ohm reading, though. That is certainly causal to poor battery charging.
Can you solder, or know the principles of creating a good solder joint?
Could be that the solder you filed down, left the material too thin. Then, fatigue from vibration and heat cycles created a crack or open air that join. Another possibility has to do with the physical connection. The wire should be coated with a thin insulation. This must be skinned back to expose the metal core of the wire. I've seen wire that were not skinned back far enough or not at all. At a bubble of solder made a good connection by connecting to the very end of the wire. If the solder "bubble" was filed off the end, then the remaining wire insulation prevents or reduces electrical contact, even though the solder is snuggled up right next to it. These things are sometimes hard to see with the naked eye. And, optical magnification is need to see the problem. Or, the wire is extracted, skinned, tinned, and re-soldered to the cleaned terminal. Bear in mind I haven't seen a CB650 rotor, and don't know the specific arrangement. But, I have seen many similar devices, motors, generators, alternators that use similar connection techniques. Higher resistance reading are frequently attributed to bad, poor, or "cold" solder joints.
Then again, you may have simply accumulated a glaze over the slip ring surface. You should also check the area for oil contamination, which may have burned an insulative glaze onto the slip rings.
