I've just updated the solution page. Previously, all the solutions were listed together, which made it a bit difficult to navigate. There was a limit of 5 solutions per level (an attempt to try to keep the main page usable), which is why some solutions got automatically removed to make way for others.
I've redesigned the solution page, which now lists just the best solutions for each level and also shows the remaining money for each solution. You can still see all the solutions for a particular level by going to another page (by clicking on the level name).
I've removed the 5 solutions per level limit, because the main page won't get overcrowded this way.
Previously you could rate solutions, but because there were so many this didn't seem to be workable. I've removed the ratings from the current solution page (the solutions are ranked by score - money remaining).
To answer the original question - I think modifications are allowed, but I think the original author would probably appreciate a mention in the 'description' field.
I hope this is a welcome improvement. As always, comments/criticisms are gratefully appreciated.