I'm not being a dick. I'm letting you know as a trained and qualified motorcycle mechanic, it is in my professional and knowledgeable opinion that you aren't going to be able to repair that thing for anything reasonable, nor are you going to easily swap something in there for less than the price of buying something newer. When I was working at the bike shop, literally 90% of the pre-'00 bikes that came in were bikes that people had bought from someone for an 'awesome deal' and it only needed 'a little bit of work', and they just wanted to get it 'freshened up'. Those bikes also usually ended up needing thousands of dollars in repairs to make them SAFE to ride, much less run/ride anything near decent.
The 2 things that let me know without a doubt that it's junk are:
1. it's $300
2. it's an 85 Honda
Honda bikes aren't like Honda cars. Honda bikes are pretty much crap. Shit, even the newer ones are junk.
Honestly dude, you're going to end up dumping a LOT more money in that bike than you're anticipating. Use the money elsewhere.
How do the tires look? How are the brakes, front and rear? What kind of condition are the chain and sprockets in? How long has it sat up? Why does the tank need to be resealed? Does it have any engine oil leaks? When was the last time the oil and/or coolant was changed? What kinda condition is the battery in? Are all the cables for the choke, throttle, and clutch in good condition? Are all the fuel and vaccum lines new, or at least recently changed?
I would almost bet money that EVERYTHING I just listed is NOT in proper working order. All of that is stuff that needs to be in VERY GOOD condition in order for the bike to be safe to ride. Bikes aren't like cars, if you leak a little oil from the bike, where's it gonna go? All over the rear brakes and the rear tire. Ever ride a bike that the throttle sticks wide open because the cables are old and coming apart? They said the bike is in good running condition, but the tank needs to be resealed...why on earth would they drive a bike around with a leaky gas tank? What all has the gas leaked on and ruined? Keep in mind if it has been sitting for more than a few weeks, the carbs are probably starting to get gummed up and will more than likely need a good cleaning. For that matter, what kinda condition are the diaphragms in the carbs in? I've never seen a 25 year old diaphragm be in good condition.
And guess how much all the stuff above is going to cost you? An arm and a fucking leg, or what you could have spent on getting a decent bike in the first place.
There's no such thing as 'just spending $300 on an old bike'.
But like I said, I'm just giving you my professional opinion based on what I know and what I've seen.