I have the second of those two engines (Fast trails) in my madass, the YX160, and I do have the 4 valve head. It was in the bike when I bought it.
This engine is kickstart only, with no place to put an electric start.
It does run very well, and kickstarts in 1-2 kicks. Top speed with a 150lb driver could be over 70mph, depending on which head, exhaust, and cam etc.. you use. I am 195lbs and with 15/39 gearing I can get 64-65mph, but rarely go above 50-55. It says race engine, but is easily set up for the road. Shifting is 4 up, and pretty smooth. You need an Asspipe or a gutted madass 125 exhaust for this motor. I use the gutted 125.
If you go with this engine, look into the 2V head, that may actually work better according to some reports.
I do not like that this engine is kickstart only. I would prefer electric start, and if this engine ever fails, may go with a 150 electric.
I do not like that the 4 valve head is a shimmed valve head. You cannot adjust the valve clearance unless you remove the head. Trying to measure the valve clearance is even difficult.
This head uses the smallest spark plug I have seen. It is an NGK ER9EH. It is removed with a 1/2" socket, and has a tiny threaded end.
Available, yes, but you won't find these plugs at your local Pep Boys. Odd plug=inconvenience, and it could probably strip more easily, although I have had no problems.