However much someone is willing to pay you for it. Seriously, with me being from Alaska, I have never head of any kind of collector market for old snowmobile's. Most people want the newest, fastest machine they can afford, and usually shun the older stuff.
this is the same question as your 64 Chrysler question with a different car. my answer is the same.
all the other responders hit one of the 2 nails to answer your question. yes, a different engine will help, but it is not a cost effective answer, as everybody else stated.
The other factor you...
Yep, your alternator is probably not making power. Pull it and replace it.
Wow, you are still driving a 1970 Opel GT. You are either fanatical about preventive maintenance or were desperate for a car when you got it.
Go American and get the Camaro. Even though the Toyota may be hot rodded to over 600 hp, there are people who are enhancing the 6.2L V-8 in the Camaro to over 1000 hp.
It all depends on your mechanical ability and how much you like the car. If you do the repair yourself, parts should cost less than $200, and your lien holder will never know the engine had problems.
That 2.0 has nothing to do with how fast it will be. It is the label for how big the engine is( in your case, a 2 liter, 4 cylinder engine). Your right foot controls how fast the car will be.