Also going into year 11, and my advice to you is to look at the past papers for the subjects you're not so sure about. If it looks interesting and like something you're willing to commit time to, then I reckon go with it.
Subjects like physics rely a lot on strong foundations, so you need to make sure that you learn the basics of it really well. I presume you're strong in maths because you're doing Ext1 maths, so that part of physics shouldn't be too much of a problem. I don't know anything about IPT because my school doesn't offer VET courses, but if you prefer that course and think you'd do better, I reckon change it.
With modern history vs engineering, I considered doing both of these subjects, however in my particular circumstance it was too difficult for me to do engineering because my school doesn't offer it on campus, so you'd have to travel once a week to go to a 4/5 hour lesson, and for me it wasn't worth all that effort. With engineering, you have to consider several things, such as the fact that if you do it in year 12, I think you have to do a major work. From my understanding the scaling isn't great for it either, however if you truly love engineering, it's worth doing it. I know a few people who are doing it, and when I did consider doing it I thought that the course was super interesting.
Modern history requires you to have strong historical skills like any other history subject, so if you don't have super strong skills, or aren't keen on the skills-based aspect of history, I'd consider changing it.
All in all, I don't know your individual situation, and only you know what's best for yourself, so take your time to think about this and make sure you know what you'd prefer.
