Great advice from Jack. I have an electric LT-25 Kadet, just a little bigger in span and wing area but a bit lighter than yours at about 70oz. I wouldn't worry too much about yours weighing 6lb - there is plenty of history of LT-25s flying really well at around that weight, on power systems not as good as we have access to today.
Mine has flown well on as little as 240W (4S, ~400Kv, 13x10) and is very sporty on 480W (6S, 410Kv, 12x10).The 6S are only 2200mAH, but I get easily 10 minutes using a bit over half the pack, and at only 24A max, everything is understressed.
If you can comfortably fit a 12" prop, then I'd target a motor around 650-700Kv for 4S and 450-500Kv for 6S. Even at 6lb, that will still give you well over 80W/lb and a well matched efficient system that will get your Kadet up and away very well. Conveniently, many motors in that Kv range tend to weigh around 10oz, so that will help balance the Fox 45 you're removing.
If you have to go with a smaller prop, then you'll need a bit higher Kv - If you take a look here
, you'll find a really easy to use tool that lets you trade off maximum prop sizes, cell count and Kv without needing to know all sorts of motor details.
You can then use the compare feature on Hobby King's website to find motors that match your desired Kv and weight.
6s might sound a bit extreme, but with cheap ESCs like the Plush60 that handle higher voltage, the cost disadvantage of higher voltage has gone away, and the electrical efficiency benefits are compelling.
But first things first, figure out what diameter prop you're happy with (enough ground clearance) and work from there.