You can make up your own template by tracing the shape of a cradle from a twin pipe or trellis frame onto card board (breakfast cereal package) ..... then transferring it to steel ...... The biggest problem people at home with no oxy cutting gear will have is cutting out the curved top side since there's a HEAP of metal to remove if you want to cut it in one piece from a single plate .
The answer there is to make each side of the cradle in TWO pieces so that you can cut them out of cheap and easy to obtain .... FLAT BAR .... IF you make each side in two parts you can shape the metal using grinders ... then mate and weld them together ...... I'd shape the top part that attaches to the frame exactly the same shape as the original frame (unless you can leave additional metal on and clear the rear of the engine) then drill three holes into it ... one for the swingarm pivot tube , one for the brake lever pivot / chain roller shaft .... and the third for the rear engine mount bolt .... ( that's why you make them out of cardboard first ... to get the holes right ) ...
The MAIN thing you need to make sure of is that the front of the cradle sits flush against the underside of the engine ...... so that the top two engine mount holes are aligned .... You DON'T want to have the cradle pulling on the engine cases and top mount (pre loading them) when the bolts are tightened ..... And IF the cradle has ANY up and down movement in it at all ..... then it'll be doing diddly squat to take the load off the top engine mount ....
And as to lengthening a frame .... the last place you'd cut it is in the middle where the most stress is concentrated (Snitchy's son learnt that first hand) ..... The best place would be around an inch or two from the steering head ..... then you'd use pipe inside the stock pipe and gussets cut from pipe to go over the outside of the stock pipe ... do the welding in short stages (back step skip sequence stitch welding) so as to not over heat and weaken the parent metal (original frame structure) ..... Anywhere that has welds ACROSS the the width of the tube will need to be gussetted or it WILL be severely weakened ....
TIG welding would be the best since it has the lowest amount of heat transfer ..... but good MIG welding can also minimise heat transfer if you do quick small welds ..... and you can also do stick welds in stages to minimise the heat spread ...
