I agree with ckau- these Howhit's are solid and reliable, don't know where you'd heard they weren't good! The GY6 is used in a lots of vehicals and if it wasn't any good they wouldn't sell or have so many modification parts. I did a simple thread on putting an ATV engine in a buggy and have transplanted a ATV engine into my wife's Yerfdog if you want idea on how to do it:
http://www.buggymasters.com/forum/showthread.php?t=1602
http://www.buggymasters.com/forum/showthread.php?t=1159
I didn't change from a GY6 because of failure but as a means to get more power and a transmission.