In regards to the clear mechbox for that MP5, the majority of clear mechboxed guns Ive had the chance to look at have mechboxes that are smaller than that of the standard sized mechbox.
Would you be able to provided any pictures at all?
If the dimensions are the same, then I suppose you are good to go, pending if the motor you currently have is able to crank the piston in the mechbox you linked. However since that is a mechbox for an M4, you will require to change out the selector plate to one for an MP5.
"Its only a little bit on fire"