I've read forum posts showing people using cut-and-reweld, machined parts, additional sections of tubing, etc. to lengthen and re-shape a cradle to make it work. This seems like a lot of work to avoid finding someone to bend you some new tubes. On a 125, the lower cradle requires so much modification that I decided that it would be much cleaner to cut it out completely and have tubes bent to replace them. I called around to fab shops in the phone book and found a shop that can bend and weld aluminum tubing, who I will use when it is time to make new cradle tubes.
I pulled off most the parts leaving the sub-frame and handle bars so that I could stand it upside down.I used a reciprocating saw to make the first frame cuts. many people use die grinders, but I was advised to use abrasive cutting methods as little as possible because they embed junk in the aluminum which can cause problems during welding. That being said, I don't think I'll be able to do all the cuts with my saws-all.
As you make cuts, be sure not to cut to close to the frame leaving some of the weld on the frame. This is to prevent cutting into the structural meat of the frame. The welds can be carefully ground off later without damaging the frame. First we cut off the coil mounts and save them. We will likely weld them back on later in a different spot.
Next we cut the bottom weld on the lower radiator mount, and bend the mount out away from the frame. I'm fairly certain that we will end up cutting off all four mounts to reposition the radiators, but I'm going to wait until I have everything else in place in case I can leave them where they are.
Next we cut along the welds just below the bottom of the Y-piece from the back toward the outside. Be careful to be sure the blade is cutting perpendicular on both sides of the square down-tube.
Now we can cut along the welds of the front of the Y-piece.
Now that the Y-piece is loose, we can cut the back of the cradle. Cut within a few inches of the forging where the tube inserts. Later we will cut the welds and remove the square tube in preparation for inserting the new tubes.
The cradle should now be loose and can be removed. Save the cradle because we may cut the engine mounts off of it to re-use. All our carefull cutting may not be necessary; depending on how we do the Y-piece, but I figure it is easier to cut more off than to put material back on later.
Now we can slip the engine in the frame and get ready to start positioning all the peripherals (radiators, tank, pipe, carb boot) before we decide exactly where the engine will be set.
I noticed that most forum posts about these conversions start with putting the engine in and building the cradel, then the rest of the post is about fighting the pipe, carb boot, fuel tank, and radiators to make them fit where the engine is located. The location of the rear swingarm mount is set in stone but the angle of the engine can be adjusted to make all these parts fit the best that they can. As I play with the angle of the engine, it is apparent that small changes in position will make a big difference in fitting all the periferals. I think it will be easier to get all these components in place (and necessary mods made) and make sure they mount up, then build the cradle as a later step.