Problem is, what H. Popkin is doing is not prewar Ueshiba aikido. Roppokai is an off-shoot of Kodokai and the Kodokai people are pretty adamant that what they do is quite different from what Ueshiba did.
Hmmm ... should have detailed the logic train a bit better, I guess. Pre-war "aikido" is really Daito ryu. Every pre-war student's art looks different. I'm told every Daito ryu school has a different look. Hence, if you're interested in pre-war "aikido", then really, try Daito ryu. And if every person's take on that training is different, does it matter which one? Yes and no. So, for my view, I've been to one seminar of Howard Popkin. I'd recommend him and I did.
Now, if you want to train in something that has the look of what Ueshiba was doing pre-war, okay, I wouldn't know. I'd still probably say try Daito ryu. And if you suggest the Takumakai, I guess the problem would be finding a local branch or dojo.
Course, if you want something with an "aikido" brand on it, then that'd probably be something from Tomiki, Shioda, or Mochizuki.