https://www.ebay.com/itm/Engine-110c...-/401134105326
No gear shifter, my son has one on a coolster 4 wheeler. It wouldn't surprise me the least to learn some company slapped them in buggies but his engine sure looks a lot like the one in my son's 4 wheeler.
As far as I know all 125ccs have reverse. In turn they have a forward and reverse shifter. From what I've looked into it the 125 is a direct bolt in swap on the atv I just have to cut out some of the body plastic to fit the shifter (there are molded cut lines). Personally I'd venture to say if they swap on the atv as easy they should on the buggy. Will have to find or make the required shift linkage.
You might search around for the used atvs in your area if you find out indeed the engines swap. I found my sons for $50. The idiot POs let the battery drain and then burnt the starter up trying to crank it. Since it was "POS chinese crap" they just wanted it out of their yard. $20 starter and a $20 battery later my son has been dogging on it for 3 years now trouble free. He is getting big enough now reverse and some more CCs would be nice is the only reason I was toying with the idea of engine swapping.
this is the 3 speed with reverse version
https://www.ebay.com/itm/125CC-SEMI-....c100505.m3226
They made a top mounted and bottom mounted starter version. My very limited research the engines will swap out but if you want to keep your old engine to use some parts on the new one as needed they do not interchange as easily. I want to say the bottom mounted version was an earlier design and you don't see as many of them. Been awhile since I looked into it.