For the Jumbo, it apparently had a 6.0L Chevrolet V8 engine, but not much else is known. At least this article has plenty of images and one of the photos has a couple more informations on the engine in its description.
For the Manta A, that would be another interesting project. It had a racing version aswell, which ran in two very different series, IMSA/ETCC in GTU and Gr.2 configurations and a Scandinavian Super Silhouette championship as Gr.5 specification.
This is the Gr.5 car. From what I could gather on this site ( http://rejsa.nu/forum/viewtopic.php?p=876129&sid=f07de43f68a706e734b6baa91028887e ), it was powered by a BMW M1 engine (I assume the Procar/Gr.4 engine). Although I have my doubts on it being an A, as the general shape looks "suspiciously" similar to a B instead.
The IMSA/ETCC car. It seems that the A shared the same L4 1.9L engine of the B.
It also raced in Trans-Am, although with it had wider fenders and a little spoiler.