Should clarify a few things here...
The Menatone I suggested cops the JTM45 very well. Won't do plexi/JCM...
The ZVex and MI Audio are more modern JCM800 sounding but don't get that vintage JTM sound.
Given how much I like Menatones (I own 3, have tried more) perhaps try a King Of The Britains. I haven't tried it myself, but the latest version has a switch for Plexi and JTM voicings. Here's one with a tele demo.