Cool down little Frankie! I know very well, that fighting with such problems are sometimes very hard (i have to do such every day in my work with electrics...)....

I want to give next advice: Put both Ampere meter(in ab.20A scale) to those wires, what are going to battery from regualtor AND also one Voltage meter to battery terminals.... Run bike from idle to higher RPMs(in street) and MAKE NOTES, what is going on.... If battery is in normal condition(!!!! Did U check it in close car battery maintain place?), then starting of bike will take some current from battery=> Voltage goes lower, maybe under 12.5V. When bike reach normal temperature, battery will be charged enough, then voltage should be over 12.5V!!! Then start drive and look then voltage: it should go upper and stay between 12.5-14.7V and current goes whole time lower, when battery charge more and battery voltage goes up to end (ab.14.5V)..... If all function, like shopuld with ampre and volate meters, then You can forget 120% that warning light !!!!! But if it doesnt charge enough(voltage and Ampres), then You have failed regulator..... I know these thiongs are not so simple to understand, but measurements will show what is wrong... As they say in Electrex fault finding diagram, sometimes it needs MORE measurements !!! Only ampere meter and voltage meter will show, is it REALLY charhing, or no. IF NO, then You have to check all connectors and maybe change regulator....
Come to here with Your bike and we put meters to Your bike and "listen" what is going on...
Is your battery 100% ok?
U told, that You cleaned and welded some connectors, are U 100% sure, that ALL was done?
It can also be, that stator have failed, that it dont give enough power(current) to regulator. that can measured by some insulation and resistance measurements....
I am little drunked, so, i cant give full quarantee, what i have written, sorry...

Anyway You are welcome to Finland with Your 907 charge problems
At end: If it charge battery, then DONT WORRY about warning light and then that warning light "feed" from regulator have failed......