Space Bug Jr. blue tank

Martin Hepperle makes the following entry regarding 1955 Space Bug Jrs. :

A Space Bug Jr. with a blue fuel tank. A Space Bug Jr. with a cream fuel tank.

Space Bug Jr. .049 (0.8 cm³)

Manufactured circa in 1955 (#370)

These engines already have the new single piece piston and cylinder design, which was used for the next 50 years on all Cox engines. Also the reed valve assembly was simplified and now used only one reed of 0.13 mm (1/200") thickness. Finally, the glow heads have the large thread which became Cox standard for the next 50 years. The remaining parts are identical, except for the colored fuel tanks.

The color was usually chosen to match the firewall of the models equipped with these engines (TD-3, Prop Rod, Water Wizard).

I am still looking for a sample with a black plastic tank.  [end of entry]

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I acquired a blue tank Space Bug Jr. with a bozo plier mutilated old style cylinder/head/piston, so I am tempted to sub a clean early Baby Bee top end to cobble/restore this otherwise good looking specimen. Would that be kosher?

Sure, they were the same parts. Look for proper plug, see photo.
The white SBJ was the first, followed by red and yellow, then the blue version. The white tank version was first sold with a prop nut instead of the spinner, some say the spinner was designed to pinch with fingers to help stop the engine and if i'm not mistaken i think i had read that in a Cox engine manual?