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If your using metal mounts drilled and tapped for your Fora I believe the spacing is 15mm on the engine lugs . In New Zealand years ago a very common speed limit engine was the ASP / Magnum.15 using those mounts and F2D models . I believe a FP,La ,or TT .15 will fit also . Just not as fast .
Try an ASP 7x4 to start . The balance might be off so start with the engine pulled all the way out . Nose heavy = dull ... tail heavy = whoa !
Brad