First of all, it's looking great to me! As I see it, the problem with "pure" G1 style is that there are actually several G1 styles. I did a generation drawing meme a couple of years ago and was surprised to find there were at least three different styles in the cartoons alone (I'm not too familiar with the G1 cartoons but apparently the style in the Friends series was different from the
Rescue at Midnight Castle, and of course the MLP Tales style is very different from the earlier cartoons), and the art of the backcards and comics are also somewhat different from each other and from the cartoon styles. Phew!
That said, I think you're coming very close to the G1 cartoon styles! Their anatomy certainly has a G1 look, and the eye and mouth shapes remind me of the cartoons. There's one small anatomy detail I would like to point out, though, something that seems to be quite common among pony artists: drawing the shoulder (or point of shoulder) more pronounced than it generally is in equines, making the anatomy look slightly more canine. Here it's mainly the rearing unicorn (Gypsy?) I'm thinking of. But like I said, it's a small detail, and I'm only mentioning it since you asked for feedback on a purely G1 style - it's common in fan art and may be a deliberate stylistic choice but I don't recall seeing it in any of the Hasbro art styles nor the toys. Making the necks a tad longer and leaving the shoulder less defined should help.
Sorry, this is getting long! I just have to say I love the interaction between the ponies and their facial expressions that you can already see very well even if it's only a lineart.
For some reason I really love how you've drawn their legs (random, I know!)... and Zig Zag looks simply adorable. Very much looking forward to seeing this finished!