You need a light weight engine in your Fok D.VII??? The original 185hp BMW is about 650 lbs, maybe more. With that weight budget, you could use a 300ci Ford I6 truck engine & some lead bars? Am I wrong?
Oh, you're building a Baslee 80% Fok D7 too? One of my friends is using a belt-drive VW in his D7 and another friend is thinking of using the grear-drive Geo in his D7. I think its a Geo 4-cylinder version.
PS I don't think you can build one at 550, think more like 600-700
Why a Miata engine? That inline, liquid cooled 4 cyl engine is not in any way light. I could see you going to a Mazda rotary for this project in order to save weight, but not an in-line 4 with associated heavy cooling.
Of note, a Corvair engine will be about 230lbs installed. That might be a better alternative, and will make a very honest 100hp.