there is nothing wrong with modifying, as long as you can weld good, if you machine a coupler for every part you butt, and weld good, it probbaly will be stronger than a factory piece of tubing. try to put the butts were a diagonal brace comes in as well, and don't butt weld a tube if the whole piece can be replaced easily.
and for sprocket gearing, the best for a bike
Engine is about 7:1, it will top about 90 mph, and go 0-60 faster than you can shift.
just my opinion, and yes i am currently building a mc powered car from ground up!!
this is how i would mod that car. tho i would rebuild the chassis all together, with some tubing that had a wall to it, i think that car is like .o58 tubing, and regardless of what some say dom tubing is the only way to go, no erw. and also use the oddy parts for drive etc.