Me and Ryan dedpead were making rollers for stock but they suck tbh. They weigh a ton, they are a bad design, they have very little range and the amount of work it takes to make them perform less bad can be spent getting a good variator tuned perfecto.
There are a lot of good options for aftermarket variator, I made my own but I had some issues with production, might make them again someday, 40 of them are floating around out in the world, haha, the 2 I have on my own bikes still work great.
The dos looks good but I haven't run it, it's the same scooter based idea I had.
The polini is kinda the gold standard, it seems to go right on and work out of the box on just about everything.
The malossi, I tried one back when I really didn't know what I was doing. I think it's probably fine but on a lower performing setup the out of the box weighs were too light and I spent a lot of time messing with timing and jetting and generally being a doofus.
Then there are a few new options I don't even know about so hopefully others will chime in.
If you're in a budget, I would seriously consider the dos and order a big bag of cheap scooter weights. It's literally night and day having a good variator, plus the bigger range is like getting speed gears without losing low end.
Especially on the kits like malossi and Olympia that don't make as much top end. My cousin's bravo that I built him with a 43 Olympia/cut crank/ 12:12/ Tecno circuit was barely doing 40 with the stock vario and putting the MF vario on it got it to almost 45 (46 downhill!) And it trucks getting there. That kit doesn't really rev out so the bigger range makes a huge difference.