The Robin Hood is much different in size and impedance, hence a re-design - mechanical and electrical - would be called for to obtain optimal results. And frankly, the TFA-2004Jr is a superior design IMHO.
The TFA-2004 Senior could be a step up - hard to believe since by the numbers the Junior is plenty adequate for a 45 (it can handle a 300B quite well ...) and the Junior should have slightly better treble. For a 45, there is not much point in getting anything less than the full-nickel core though. But having heard a couple John Tucker 45 amps with nickel-core TFA-2004s, I am confident there is an improvement. It does fit the chassis, though you would have to drill new mounting holes and a grommeted hole for the leads.