Look for the Basilisk II 0.9 JIT Mac emulator on google. I don't remember where I got it but it should be easy to find. Once you get that working, PM me and I'll give you a hard drive volume with System 7.6.1 (or you can make your own from the disk images on the Apple website) and a ROM, if you can't find one. The Mac Lemmings available on HOTU has a corrupted file that breaks most of the music, but I have the original game so I can just include that into the volume along with my custom level files.
If you want to try your hand with the editor, it's an old DOS program called Lemedit. You need a special version of it called Lemedit3 that works in Dosbox. To import the levels into Basilisk and Lemmings, you need HFV Explorer (easy to find) and an older version of a program called Resorcerer, version 4 something that works on System 7.x. This took me forever to find - I think I finally got it from some obscure Japanese site - but it allows you to replace the existing levels with your own after a few minor hacks.
Basilisk generally runs very well for me (although on a 3ghz single core opteron), usually faster than my old Mac and with superior sound quality. A few games exhibit slightly stuttering music, particularly the Lucasarts SCUMM games, but Lemmings works pretty much perfectly. I played through the entire game and the ONML expansion pack just two weeks ago. The emulator seems to take advantage of dual cores, so that should also help if you have one.
