Its not worth the bother trying to find a new set of forks, a brake caliper, a new wheel with a disk and a master cylinder, you'll soo be 17 and then it's goodbye to DT50 and hello something better.
Your DT50 is such a lightweight bike the drum brake should have no trouble coping, pull your front wheel off and buy some brake/clutch cleaner from your local motor shop, give the drum a good spray and wipe out, you may notice a small raised ridge that has built up on the edge of the drum, get some coarse wet/dry and remove this ridge, grease the brake camshaft and remove any excess grease...and finally buy yourself some grooved EBC shoes from Wemoto, youl be amazed how good your little drum can be, my 175 has got drums and ive never had a problem in the 25 years ive had it!!