Good results
I do not agree with 4dtvman. The DJ drive is definately worth it. When getting on a highway or at speeds over 60 one seems to want another gear. The DJ mod makes that "feel" happen. As far as the bottom end, just give it more right hand and it works just fine. It also "smoothes" out the whole feeling of a C50. The new 5 gears are really spread out so it makes it feel like 6 gears. It looks just like the stock C50 drive you took off. If an M50 has a black drive, (dunno) then it will be replaced with a silver color drive.
As a byproduct, you may get about 14% better mpgs - I have on occaision gotten 57-58 mpg. I rarely drop below 52 mpg in combined, mostly city, riding. Your right hand has a great deal of influence on your mpgs. Vibrations and engine working at fwy speeds is greatly reduced. This aspect also reduces rider fatigue.
Installation is simple. If you know how to take the rear wheel off you can do it. It is very helpful to have a MC jack. I chose the "file the holes" elongated instead of the drill with a 9/16 drill bit - it leaves more metal. There are articles at VR about various methods. Even with a few tries and pulling the U-Joint out (put it back in with a broom handle as noted in the instructions), it only took about 1.5 hrs to install. Have a new cotter pin handy. Drain whatever oil is in the drive and fill it with new Amsoil or whatever you like - just make sure it is correctly filled.
There are others who are just selling the C90 drive, then you have to modify it. Some others have not done as much testing or research and there have been some expensive problems. Use your own judgement on your own skill/fabrication levels. The DJ drive is a bolt-on mod and makes the bike so much better. I was surprised that an M50 bought one - I thought those would want to retain their "zoomie" aspect. I left off the fake swing arm - it doesn't really do anything. A couple of pounds off makes up for more junk bolted on the bike - every 6 pounds is worth 1 HP.
I am surprised at the general "Negative Response" from members here on this board. There are some naysayers on the VR board, but in general, most of the comments and results are "Positive Responses". Maybe I am mistaken in this, but it seems that way. Oh well, make your own choice.
I won't post any links, just go find it yourself - I wouldn't want to upset any rules or "the old guard" here.
just my 2 rubles