Doubtful it's a zonger. The Zongshens hit the US market circa 2000, then completely vanished around `04. Other than a few direct imports, the old z125s (there were two versions) never made it here. The z110 sold for $1K and, for a Chinese knockoff motor, was the cream of the crop. By `03, the feebay race-to-the-bottom was well underway, 110-125cc lumps sometimes under $150; the names came & went within weeks. All except the Lifans turned out to be complete junk. AFAIK the first of the Lifan lineup didn't reach our market until after the Zongers were gone, with virtually no overlap.
TRG is right, there's been a lot of fast & loose played with the nomenclature. That said, I seriously doubt that you have a zonger. I believe that there's a z155 that was (is?) available, beginning a few years ago, not really sure about the precise timeline. However, it doesn't seem to have gotten any marketplace traction and it cost more $250; I do recall a handful of really mixed reviews. Over the past 10-12 years Lifans have dominated their market, with Jialing being the only other choice, sold under various monikers including YX, GPX, Daytona, etc. At least some L110/125 motors have the screw-in type tappet covers. All that we know, to a certainty, (based on the SN) is that those cases were destined for a motor with a 54mm bore. It doesn't indicate anything else, like which head, crank, clutch, tranny was factory-installed. That number tag was a paper decal, long gone. At one time, a number of fleebay-by-night sellers cobbled-together oddball engines using surplus/odd lot parts, including case halves. IMO, yours is too new to worry about this.
That welding is far fuglier than I had imagined. IMHO, that countershaft is scrap metal. TRG is right about the clutch baskets. They copied the Honda Nice/Wave design...loosely....kindasorta; in a word, poorly. The Honda clutch basket used damping springs, which don't fail but, those cost more than rubber pucks.
They're made from hardened steel and hardened steel, for practical purposes of this thread discussion, is virtually impossible to weld to a sprocket. That's why the weld didn't "stick", the melting point of the C/S wasn't reached. It also proves that the splines didn't become stripped from riding; they were ground off...a numbnuts move. There are other, basic, reasons why this is unsound practice. 
