Airports wise, you have a choice of Leeds/Bradford, Humberside or Doncaster Robin Hood. Doncaster Robin Hood is probably the closest and possibly the easiest to get to Cadwell Park from.
As for getting there, you could either hire a car and drive or get a train from Doncaster to Louth. However the car option would obviously be easier.
Accomodation wise, your going to be hard pushed to find anywhere local to Cadwell Park to stay by now. The local campsites are all fully booked too. If your happy to book a hotel then there are a few choices within driving distance…Travelodge/Premier Inn have a few options in the Boston or Lincoln area. Another option is a hotel Ive used a few times called ‘The Lincolnshire Oak’. Its a nice quiet hotel in a town called Sleaford, competative prices, good bar and food but relatively basic rooms.
In summary, if it was me making your journey, I’d fly into Doncaster Robin Hood, hire a car and drive to the Premier Inn in Lincoln. Id then use this as a base for both Cadwell Park and to visit some of the local attractions.
(another option would be to look at staying somewhere commercial like Butlins in Skegness or Haven in Mablethorpe. These are both less than 20 miles from Cadwell Park. There are also a large amount of other Holiday sites and guest houses along the coast between Skegness and Mablethorpe)
I hope this helps
Any more info you require, please feel free to ask. I was local to the area for quite a while so I know the area very very well.