The HRM/PMR jackshaft kit is $120, some people have had the plate bend under load (power) although I doubt 2-3 hp would bend it. Yes, it provides options in gearing, trading top speed (23 mph stock per MM's specs, so I would expect maybe 15-20 mph with a noticably higher ratio) for increased torque at the wheel. The jackshaft adds complexity and increases parasitic loss. The BMI jackshaft plate with hub is $39 but doesn't include the shaft and sprockets. A 212 Predator is $100 (add $25-50 for a good clutch, a Max Torque SS 3/4" #35 12T is $27 from BMI, 13T is $36, 15T is $48) and makes double the power, or more, of the MM Lifan 80/105 and won't need the added complexity of the jackshaft. I once saw dyno charts for stock 212 Predators, 7 hp for the non-hemi, 8 hp for the hemi, both rated 6.5 hp. I've tried to find the article with the dynamometer results but don't remember where/how I found it, probably stumbled upon it. If you want to stay with a Lifan engine, you can get a Lifan 212 for $139 from HD, regularly $189. Lifan claims their engines are assembled using "automotive grade" parts (crank, rod, piston, rings), I don't know whether they are higher quality than Predators and/or other clones, or not. You can limit the throttle opening on the 212 for beginners/young riders and increase it as experience/skill/age increases. No doubt in my mind, I'd choose the 212 swap. You may or may not need a new chain depending on how the current engine is mounted (forward/rearward), the current position of the tensioner and the tooth count of the new clutch sprocket, the crank would probably be a little higher on the 212. I'm not recommending a torque converter instead of a clutch because the small frame MMs require frame modification to fit a TC, something not practical for everyone.
https://www.bmikarts.com/Max-Torque...-a-34-Bore-for-Go-Kart-or-Minibike_p_721.html
https://www.bmikarts.com/Max-Torque-SS-Clutch-34-Bore-35-Chain-13T-or-15T_p_4679.html
https://www.homedepot.com/p/LIFAN-7...t-Recoil-Start-Gas-Engine-LF170F-BQ/205819157
https://www.oldminibikes.com/forum/...ith-predator-212-and-torque-converter.103977/
https://www.oldminibikes.com/forum/index.php?search/104560/&q=monster+moto&c[title_only]=1&o=date
https://www.oldminibikes.com/forum/index.php?search/104599/&q=mega+moto&c[title_only]=1&o=date