Well..sometimes it just doesn't move and it clicks under heavy load
What i would do in your position is just have realistic expectations. It sounds like the possibilities for your machine started in the 'low-to-medium' range and might have gone down to 'really low' if it makes a clicking noise and doesn't move. That's not engine or belts.. almost certainly internal to trans. Might have some already partially stripped teeth skipping across each other as the shafts flex apart from each other, etc.
The good thing, things that 'look like a riding mower', most people can't tell the difference between one with a lot of pulling potential and one without, so you can often find a 'good starting point' for pretty cheap because the person who has it might think it's 'just another riding mower'.
So i would keep your eyes out for the next project with better bones for pulling. Anything you learn on the current machine along the way will be put to use again in the future, so as long as you're not sinking a crazy amount of time or money, or getting demotivated and giving up, it's all good in the end.