Is your carburetor clean? Using a fuel filter? I had no problem going 80+ (indicated) at 215lbs. I've got a 14/30T sprocket setup. I'm 200lbs now, and regularly find myself passing people at 80+ mph on accident (I need to keep an eye on that!).
Before boring out, check your carb, check your fuel, make sure your exhaust system is good (rust? holes? exhaust studs still there and working well? bolted on tight?). You can clean your carb yourself, check your exhaust yourself, etc. If there's holes in the exhaust system, you can use muffler tape (get the high temp kind) to fix it if you're not concerned about being pretty. Also, you'd spend less on buying a whole new exhaust system than you would on re-boring.
You should be flying down the expressway if everything else is good.
Check out wrz's bike buying checklist
, too, while you're at it.
Have you been on the bike before? Have you ridden before?
An 87 is good because there's a ton of parts out there.
Plus there's so many things that you can do with a Rebel.