It all depends on what color the plugs on your servos are? If they are black and have three splines on them the the answer is no you cannot use any modern servo in that receiver and switch harness.

If your existing servos have blue plastic plugs the the answer is Yes you can use any modern servo even Futaba if you shave off that little spline. Also you can use analog as well as digital but P.S. digital is a waste of money when you are learning and on most trainers.

Hint since you are on limited funds, run don't walk to your nearest RC club now. You will find an invaluable resource for 'stuff' and usually a much better prices than the internet. This especially so for new follks any age many in your local club tend to help support new folks when they become menbers with donation of bits and pieces and whole airplane set ups at times.

John