I think you need to define Aiki and internal.
"Big" question. I'll try to sketch out ...
As far as I understand and try to practice aiki
, it has got two main aspects:
First aspect is to organise oneself, body and feeling (kimochi) to get an integrated unity. This aspect of aiki can be practiced and developed through solo exercises.
Second aspect is connecting one's own body and feeling to another body by what we call "atari" (engegement, connection). And then to affect and controll the structure (not the movement) of this other body.
The first aspect is required to make the second aspect work.
(I'm not talking about "IS" because this to me seems to be a specific, certain way of internal work.) to me means using not outer movements, but one's own body structure. In an extreme case there will be no visible movement of tori but uke's structure is affected nevertheless. The work is done within the body. (This doesn't mean at all that you don't move anymore!)
How this is done, there are different methods, ways: It may start with using the internal deep musculature whithin the torso and within the extremities, muscle we often are not aware of. Then it means using visualization which helps to bulid up and use the internal structure of our body. The using ki control, but in a different way than it is understood and done in ki-aikido as far es I knwo. Using in and yo aspects, like it also can be found in koryu. .... Things like that.
This in my eyes - even if I am not very skilfull and even if my describing is worse and completely incomplete - is the essence of how aikido works as far as I understand it. There is a lot to learn after, whithin and behind waza. And I don't know, how it can be done otherwise.
Is this more clear?