Abdelhub - Gotta love those old Greeves, and they'd certainly get the job done. I remember them well, and about the nicest anyone said about their appearance was "cobbly".
Rpage11 - A good friend had a Penton 125 and I got to ride it a bit many years ago. I loved it but just couldn't afford the price or I would haved owned one for sure. You're a lucky guy to have one, and it looks to be in great shape.
Eyedragaknee - that's probably very close to the perfect deep woods bike, especially if you could fit it with a larger tank. How hard is it to get plated?
Mudgepondexpress - got to agree that if we're sticking to old school the Yamaha DT-175 or even the older CT-175 would fill the bill nicely. I'm thinking an old Bultaco Matador or Alpina would work really well too.