Big things would be Advertising separate from other things and bound to only advertising, and require it contract with the other units on public and FRAND terms. Web Search would be another unit, and it would be barred from developing its own advertising platform and need to use a mix of advertising platforms based on public criteria, probably with a cap of say 75%? AdWords. You'd have at least one more group for communications (mail, the 7 messengers, etc) which maybe includes the document tools too, and might include G Suite; this group could develop its own ad platform, but not to sell ads on 3rd party sites. Android would need to be a separate unit, it could either require a per device fee or FRAND terms for search etc bundling (similar the what they do in the EU); Chrome maybe fits in this group, or may need its own group. Google Fiber would probably get shut down or sold to an incumbent telco, but maybe just spun out. Waymo and other research stuff would probably need to be spun out, not sure if that can live on its own though.

Cloud services would be its own group, perhaps providing services to the other groups, possibly requiring public pricing, but I don't know if that's really an issue.

I think that's most of it. Lawyers from DOJ and Alphabet could work out the details. Getting a competitive ad market out of the deal would be hard, but at least it could be more transparent, and eliminating cross-subsidization of Google businesses is definitely possible.

Start by cloning the whole source repository for each company, and prune out the things that don't need to stay; if in doubt all successor companies get access to all of it.