Get the longest 6mm bolts you can find that are threaded all the way up to the head, so you can use internal nuts to cinch down once the bolts are driven all the way in. Think they're 55mm long. Come in packs of four, grade 8.8, for like $1.20... perfect for the motor mounts!
They're long enough for slightly oversize frames too.
As far as the bolts you need to get assorted sizes
4) 4mm for the mag cover
The most popular size is 6mm for Clutch cover, Sprocket cover, Cases
Intake and exhaust 6mm threaded rod and possible head studs depending on engine. I have a completly different engin than most members. I have a Grubee 48cc Spitfire Roundhead, so lengths vary, but having an assortment is always a good thing. You can start with 1/2 doz of each lenghts 25, 35, 50, 75mm should be the longest you would need except for maybe the rear mount. Hope this helps.....
I would also pick up some 6mm washers, lockwasher, and nylock nuts. And some Red Loctite........
I have the 49cc engine and one of my bolts broke, I haven't looked too closely yet but I am still riding it and it seems fine. Does the 49cc take the same 6mm bolts? And can you saw them off after you have screwed them in?