AFAIK, while they supposedly made N1 harder, they lowered the passing marks to compensate. Looking at wikipedia, it used to be 70%, and it's now 100/180≈56%. If you take this into account, i.e. rate yourself by the old marks, then if you want to use the old tests as a ballpark gauge of ability, why not?

Of course, if you have access to newer practice tests, you might as well use them instead. I had some old ones, and I think the test format was a bit different as well?

Well, if nothing else, there are some sample questions on their website, although it's pretty short. Admittedly, I scored plenty worse on the test than on the sample questions, but I was in pretty bad shape at the time.

I think there were also a couple of textbooks with practice tests available... 合格できる？ Here is one for N2.

Well, that would make sense. But since we are talking about practice tests, how would you "scale" a 模試 result? ;p

Firstly, because of the change in the scoring system you can't really say clearly what % of questions you need to get right on the new test to pass. Scoring 100/180 ~ 56% does not mean that you only got 56% of questions correct. This means that you can't really grade the papers accurately by yourself.

The new test still has a lot of questions like the old test so obviously practicing old tests is helpful for those questions. There are some slightly different style questions though that you'll need to get hold of a new test to try.

I tried the new N1 after practicing old papers and one new paper. For me the old style was a bit easier in that you could pick up a lot of easy marks at the start if your kanji reading ability was good. The new one seemed to have more reading sections.

Something to aim for when taking the old tests might be to focus on getting over 70% on each of the sections, not just overall.