I've got a frame close enough to yours, it barely makes a difference. You're already missing the front fender; if you can remove and store the rear fender, you won't need to cut it. (With a fender that wide you must either remove it or cut one side away for drive chain clearance.)
It doesn't look as though chopping the frame will be necessary, even if you desire a "board-track" style. The down tube and seat tube are the two that the motor mounts onto. Yours should both be one inch diameter, making it extremely easy. On my frame my down tube is curved. Yours is straight so the engine *SHOULD* sit right where you want it (but we'll only know when the time comes).
Other changes are just cosmetic: you may want a lay-back seat post - you can swap out the original and save it, you may want the down-swept handlebars - you can flip the originals over and down or get smaller new ones, or whatever, and save the originals.
You can even do like I am, and have somebody on the forum make an in-frame tank for you. You could build it for the vintage racing look and still be able to convert it back if you desire to later. It can be done.