Modifying a 750 from a stock (abandoned bike) into a more modern bike, depends entirely upon 2 things: your disposition and your wallet.
Disposition: are you up to reading manuals, studying how things work that you may not know, and taking the lumps when you reach utter frustration. Or, do you want "easy" solutions. Do you have the discipline to clean shredded and rusty crap into a mirror finish (about 4 hours per part). Can you walk away when boiling over, then calm down, and come back and get your teeth kicked in again and again.
If not, pay someone to do it for you.
Wallet: builds and renovations are anywhere from $1,000 to $15,000 depending upon how nice you want it, what products you choose, and how much you have to have performed by outside sources. When it breaks, you buy another part. If it can't be repaired, you buy another part. If it doesn't "look" right, you'll have to buy one that does. And so on...
I tell folks who look at the builds here, or local shows, the adage I use is like the one boat owners have: B.O.A.T. = Break Out Another Thousand. Because everything on a boat to repair is at least $1,000. For me, my experience building these into custom modern bikes, it's C.B.500 = Costs 'Bout $500 for that part...
Everything that needs to be bought seems to add up to $500. Now I prefer really trick parts and bits, so my budget is very different, and yours can be much lower, and still have a great bike. But expect to spend 2x more than you think it will. And then set aside another $1,000 for unforeseen damage. Engine rebuilds alone can be $2-$5k depending upon the level of performance you chase.
I start my projects with absolute rusted yard crap because I know I'm redoing everything anyway. But, if you have something in decent or good shape, it can save you heaps, and selling off the stock parts can help fund your project. It all depends.
So, everything depends on you. What you want, what you buy, what you have, and who you are. Try it, you might really like it.
