SandS said..
you have a lot of choices there HG , depends a bit on the size required ? does it have to span a distance ?
common size in F17 Kiln dried Hard wood , 90 x 45 or bigger ... strong and stable, ok if not getting wet constantly .
other heavy stuff .... Merbau will handle more moisture may shrink a little
the ply would be ok if it was not flexing too much after you laminated it with epoxy . i don't think epoxy resin flexes very well . the beauty of epoxy is it doesn't shrink . .................. epoxy glue is awesome !!
polyester resin likes to flex . but will shrink a little .
No Sands it would not run full length just past the rear engine mount . On the steel frame in the photo where the angle Iron start to change angles
where the two bare patches are in the photo .
Donks walker has glassed engine bearers
Id build it in layer my layer with epoxy if I used ply and make a rear engine mount bulk head for extra support.
I think if I did do it I would epoxy some temp rectangular up right strips into place the same height as the original steel frame clamp them to the inner side of the steel frames both sides once the epoxy went off then remove the steel frame it would give me the original mounting plane to work off .