oooh a MUGA. should be ok. I cant imagine there is much surfacing to be saved if ita slready been dug up a bit.
overall construction is normally about 450mm thk on these. gravel over a granular sub base. you might hit a seperator geotextile when you get to the base. or hardcore.
if you dont import stuff you might not have enough for berms. alternatively dig a borrow pit in the middle of the berms. Serves as a drain hole too. I had to use bricks and any blocks for the base of my berms despite having to dig half of the track about 2ft into the ground to get it level ish.
Your one up there is at least a weeks work without muchos assistance. decent berms are a necessity. you can be more shabby with the rollers.
If you arnt willing to spend days and days digging start with a smaller loop (for an inital pump track riding fix) and keep you keen than expand at your leisure. You’ll probably get somthing more interesting than the above as a result too.