Need instruction on changing heater core
 

2 questions (maybe 3 or 4 :) ) This is a 1985 Saab 900s. I got it for my niece 2 years ago. The valve for the heater core cracked.

What's the easiest way to fix it? I found an original replacement heater core for $75.

Should I just replace the entire thing? Or can the valve just be replaced?

Also, I'm no mechanic, though I worked at an auto assembly plant for 10 years. I have taken out & installed every part you can imagine on Toyota Corollas, Pontiac Vibes, & Chevrolet Prizms. Wondering if I can replace the heater core with my basic tool set (sockets, ratchet set, screw driver, hammer, buzz saw, etc.. :) ). Okay, but seriously, I'm trying to avoid taking it 90 miles each direction to my uncles. He would have to come get it (so 4 trips totaling 360 miles). I'm also trying to avoid taking it to the shop. They won't install used parts, & the part they have isn't a direct fit. They'd have to move the gas pedal over an inch at a ct'ost totaling $620. I simply don't have that kind of money for a couple months & my niece is attached to this car. It's the manual transmission IMO.