Yes, other forks and shocks will work.
I just put a 85 YZ250 front end on my ttr225, and it works great. To do the conversion, you need the entire front end from a YZ (wheel, brakes, axle, forks, triple clamps, steering stem, bottom steering bearing). The 85 YZ250 stuff works for sure, and I believe that 85-87 YZ125/250 stuff will work; most 85ish ITs front ends; and 85-89 YZ490 stuff will work as well. None of the TTR parts, except the top steering bearing and cap, will work in the conversion. Once you get the entire YZ front end, it will bolt right in, except, you need a 3/4" spacer between the top steering bearing cap and the stem lock nut. The bottom bearing from the YZ is the same as the TTR, but you MUST use the TTR top steering stem bearing (unless you want to do a lot of extra work) I used 2 3/8" thick 1" ID stainless washers for the spacer. I also had to cut out about 3" of the fork preload spacers to get the correct ride height. You will also need to make a "new" key assembly. I used a light switch from a 03 WR. It worked pretty well.
As far as the shock, you can go works performance, or the shock from a 86-87 tt225 will work as well. It is 1/2" longer than the 99-04 TTR 225 yielding about 1"-1 1/2" more ride height. It also has more adjustability and a stiffer spring rate than the TTR shock (I haven't figured out the exact rate yet). I am also looking to see if anything else will work, as well.
I will post pics when I figure out how, and I will try to post a detailed how-to when I get some time. As far as cost, the whole conversion has cost me $350. I hope this helps ☺.