if you want to devalue your bike put the lifan in it, ive got just 88 bore up kit on my z50r and it gos hard, anything more and you have trouble keeping front wheel on the ground with such a short wheel base.
Keep it orig
From personal experiance you dont need more that a 108 kit, This thing had all the power in the world and just on the limit of unusable tbh (a little on the safer side)
If not go a gpx 125 hi comp in it but you will never ever truley use the power
Yeah if you do get a lifan 125 get the 54x54 big valve head version. They're pretty good for the price, then you can get a high comp piston and do some porting and the thing will rip!!! All I know about Jap engines is they're unaccepted by my bank account!! Unless we're talking chainsaws then its echo all the way home, best jap engines ive ever seen or used!!!
for the money, just get the gpx 125, they absolutely rip, and cost less than it would to get that sort of power from a lifan 125. if it must be a lifan, go for the big valve headed version as stoozo said