when you decide on engines you need to consider weight.try to keep finished weight under 14 pounds.i used 4 os 25's back in 1978 and fox 25 with tn carb more recently.consider a good carb with two needle carb.i think the additional weight from the four strokes may put you over the desired weight.you do not need the most powerful engines as with the 25's it is a bit overpowered .i think K&b sportster 20's would be about perfect if i could find four new ones at a reasonable price i may do a third b-24 since i have one of the new laser cut kits.may try to save up for cj retracts that are made special for this model.Anyone have a b-24 with the century jet retracts installed?