If I had to choose only between these two engines , going merely by the information and pictures provided on each website and your own personal review of the Arrow 4.5, I would choose the Arrow 4.5 as you have had personal hands on experience with it.
If you look at Fred's motor pictures this should be his finest work using the best casting and parts as it is posted on his website. if you blow the pictures up and take a close look at the detail of Fred's engine the fit and finish leaves a lot to be desired.
The casting around the cylinder cooling fins are simply horrible the CNC work on the cylinder head also leaves a lot to be desired for the price they are asking.
Keep in mind the Arrow engine pictures I wasn't able to blow up so the same could be said about their engines however since you have purchased from them before you should have an idea of what to expect.
The intake setup on the Arrow looks to be a better thought out also.
I have been on the fence myself for awhile about what engine to use
but the more 2 strokes I check out the more I like the Honda 4 stroke.
if I could find a professionally produced 2 stroke Brand name street legal engine I would go that route but it seems the Big boys have no desire to cater to the Bicycle industry.
The worse part is even the Honda motor as good as a name as it carries doesn't offer any warranty at all once you bolt it to a bicycle.
So if you go with the Arrow let us know and take some close up pictures of what you get, if the fit and finish is good enough and you are pleased with what you get I may jump onboard also.
Good luck with whatever you choose.