Lifan 125cc kits.. Any good

If you've got the fab skills, go for it. Custom builds are not as cheap and easy as kit builds, but if done correctly are worth it.

While not a common choice due to it's configuration (horizontal) there's some examples, methods & parts sources for that type engine here: http://motorbicycling.com/showthread.php?t=19630

...but, I would suggest considering carefully it's displacement, while not a high HP screamer it is a quality four stroke & the torque from the 125cc may prove... somewhat problematic with average bicycle components. Just as a thought - they've the same in 50 & 70cc variants and this type Lifan is fully cross-compatible with it's Honda equivalent (the CRF50 & etc.) so upgrading & modifying is easy-peasy, even complete top end kits if you should later change your mind

These are great engines!
We use them for years!