Jeff,

They made three basic styles of Testors/McCoy .19 engines. Since you mentioned the boxy case I believe you are talking about the last one, the heavy case black head.

The first one was the shiny case red head which also had a blue-head RC version.
The second one had a matte finish case with lightning bolts, a red head and again, with a bluehead RC version.
The third model had the heavy case and both RC and Std. versions had black heads.

The shiny case ran great but blew head gaskets now and then. I'd make aluminum gaskets and use socket head screws if I planned to run one today.

The second model (Lightning Bolt) was probably the best of the three.

The heavy case third model would be fine for collecting but is pretty heavy to use. MECOA had parts for these blackheads some time ago, might still have some.

YUP, I just checked the mecoa.com homepage and they do have parts listed for the series 21 blackhead engines.