I am not a lawyer, but the act of downloading probably isn't what's illegal. Using a ROM image without owning a [license to use a] copy is. But since you own a physical Mac Plus, even though it doesn't work, seems like it qualifies.
In order to program that machine (or the emulator), you'll need to find a language/compiler or assembler for it. There were many and varied versions of Pascal and C available for it at the time. Each language would essentially supply its own programming environment (an IDE) and library support.
If need be, I'll use CopyRom to get a rom file. that is, if the old plus ever works. I think pascal would work since I was playing this game in vMac, and as it was loading, it said "rewritten in pascal by blah blah blah....". I'll look at what other languages are available for vMac.. wait. I guess I could use any language, as long as the compiler output was in the correct format..
I found this site thats called the mini vmac applications site, and there was a disk image (among thousands of others) that said in the contents collum: blah, blah, MacPascal, blah. so I downloaded it, and shure enough, there was a MacPascal program in there. however, I have no idea how to do anything in pascal (I did get a basic hello world program running). Here is the link for the vmac apps site:
p.s I think it was intended fr people who wanted to know how to program in pascal, not to write programms in it, because there is a window for the main code, and two smaller ones for text output and graphics output that pop up when MacPascal is luanched.