These cars were running a special size .15 engine not to compare with current .15 smallblocks. I was told by a friend that by then .21 rods did fit (if you have a Novarossi or Novamega engine), maybe someone is willen to send you a used Novarossi .21 rod to give it a try.
The mounting of these engines are close to a .21 so you can give it a try if fits.
If you want new, you can try to find a RB V15 or a Novarossi/Novamega somewhere on Ebay.
Today you can find special motormounts for the Serpent 835 to fit smallblocks, I do believe they can be used as well on the Impact-2. Whith this you can mount the newer Mega ZX15
Speed is only fiction but a 130+km/h MRS4 is for real.