Welcome to the forum and I also agree you got good advise so far...
My question for your new build is do you plan on using the same engine or are you putting a new engine in this build? Are you looking for more speed or better hill climb? And do you plan on modifying the engine for more power or leaving it stock?
If you plan on getting a new engine for this build, you can't go wrong with a Dax 66cc since they're very well balanced, already have the good bearings installed as well as high grade hardware, these are the smoothest running engines you can get without spending too much extra, and the torque and power these have will make hill climbing a lot easier...
If you plan on keeping the smaller 49cc engine you can still get 66cc performance by gearing it lower and doing the mods like porting, raising the compression, and adding in a good pipe, but it's cheaper and easier just to get a 66cc and install it, not to mention parts for the 66 are generally cheaper and easier to get. But if you modify a 66 you can get some pretty impressive performance whether you're looking for more torque to get up hills better or more top end power and rpm for speed.