OP, you are caught in between. You want to do some upgrade but not enough to make the bike attractive to thieves. Understood. But the bike isn't attractive enough to anyone for you to spend any money on it. Please take no offense. That is just the case. Best bet is to ride it as is with whatever safety and functional improvement may be necessary to get it going. I wouldn't want to steal it, but thieves on the other hand have very strange standards. There is really not protection against theft except good locks and other deterrents. Ugliness is not the key.