The size and weight of the MiG-AT is not very important... the key features in this particular role would be simple and cheap to run... safe to operate and Russian.
The plane that is being replaced, the L39 can carry all sorts of weapons, but it is old and now obsolete and worst of all... foreign.
The original MiG-AT was an Advanced trainer as denoted in its title...AT = Advanced Trainer.
It missed out on that job but simplification to make it simpler and cheaper, while being safe and stable and reliable means it could have a chance at being an interim aircraft between prop and LIFT. (lead in fighter trainer).
The Yaks are clearly too expensive to buy and to operate as a replacement for the L-39, so they clearly need an extra step between prop driven Yak and twin jet expensive sophisticated Yak.
The number of engines is not that critical... learning multi engine management is a useful thing and twin engine safety is a good thing too.
That means it will not offer a lot of advantages about simplicity and cost of operations, in comparison with the yak-130...
The MiG-AT was designed and built to do the job of the Yak-130... they were competing for the same job, so of course they are going to be similar.
This new job doesn't require the same level of performance or capabilities so the old MiG-AT can be simplified and made more basic to reduce costs and make it simpler and easier to operate and maintain.
The Mig AT is more like an alternative to the yak-130, not a complementary aircraft.
They were designed and built for teh same competition, but that does not mean the MiG-ATs design can't be revised and made more suitable to the new task at hand.
I would think in terms of export a dumbed down simplified cheaper model would be rather more attractive to some countries... especially with the option for the original MiG-AT if they want some more capable versions for specific jobs as well...
The L39 has no big problems in that role... it is just that even if they substitute all the internal components with new Russian ones, the existing airframes are not eternal and have been probably extensively used.
It has two enormous problems... that it is not a Russian aircraft and it uses a foreign engine... made by Motor Sich in the Ukraine.
Since it was produced in Czech republic I do not know if Russia has all the design data and the tooling to start production of it (in a modernised version) or if would be cheaper and more sensible to think about a new aircraft in that size with the required characteristics...
Tooling up to make a 52 year old foreign plane with a foreign engine makes little sense.
Especially something they obviously couldn't export to customers without the problems of ownership coming up.
Note: being able to repair and even modernise an aircraft does not mean being able to produce it...
The cost of tooling up and manufacturing a brand new plane is the same for an old design as it is for a new one... except the MiG-AT might already have digitised plans ready for modification and use.
So I am reading that the Yak-152 is now off the table because the RED-03 engine is manufactured in Germany (although designed by a Russo/German). Its a shame they cannot have this brilliant engine manufactured in Russia under lic. Its really quite the design V12 turbo diesel with independant banks that can operate at up to FL500. Sounds like they want to go with a jet trainer at an earlier stage.
No, they are basically saying they have a lot of L39s in use still and the Yak-130 is probably too expensive and sophisticated to be the next step up from a propeller driven aircraft, so they want something to replace the L39. They are suggesting the MiG-AT as a design to base this new simpler and cheaper aircraft on... because it is a mature design that was designed for the role of training air force pilots.