Rejuvinating a vintage bike - suggestions?

[MomOnBike]

O Collective Wisdom:

I have a 1978 Raleigh Gran Prix (Walter by name) that is insisting that I Do Something for him this snowy winter, dagnabbit! So, I- - -

1) Got him out of the shed and into the basement.

2) Loosened the seatpost by means of chain lube and percussive maintenance and got the seat up to what might be a useful height - at any rate it is no longer at the absolute bottom of its range. I was surprised and pleased to find that it hadn't rusted in place. This was good. Unfortunately, the bolt holding the seat in place has gone to that great spare parts box in the sky so I need to find a replacement.

3) Ran out of inspiration. . .

So, I ask you. Now What? I'm open to suggestions.

More details; He was a 1st anniversary present from DH, he's a 10-speed, most equipment is original, and I'd like to keep as much of the original spirit of the bike as possible, if that makes sense.

Should I go with a more modern drive train? Can I do that on a budget?

My knees veto a fixie or single-speed set-up.

I'm pretty sure I'll need a new stem, as that never was quite right for me.

Or should I just clean him up and go with what I have?

What would you do?

The bike in question: