One of the biggest problems with mass-produced vintage minibikes is the quality of the workmanship, especially the welds.
Many of the vintage minis were made by large companies as a secondary product, usually following lawn mowers or other turf equipment.
Many of these companies used an 'incentive' or 'piece-rate' production pay scale...
My very first job at Cushman was welding Truckster frame tubing together to complete the frame assembly, and I needed to weld four frames per hour to meet my production quota, but if I welded six frames, I was paid for the extra work, essentially time-and-a-half, even without being on overtime, so I always attempted to produce six frames per hour.
Those were definitely not pretty welds, and I was moving that big MIG gun so fast, I'm sure I missed some spots, and I'm sure I left some splatter on a mounting surface that some assembler had to deal with later...
I like to talk to the restorers at vintage bike shows...
Their bikes are judged for authenticity...how closely they resemble the way they appeared when they left the factory, and quite often the quality of the workmanship isn't nearly as good as the restorer could do himself, or easily correct while he has the bike completely disassembled...
Sometimes it's pretty painful to leave things untouched, but if the judging is accurate, they'll have points deducted for cleaning up the messy bits...
Can't wait to see how your project turns out...I know how much work it can be to clean up a frame, and I respect your effort.