can u make units keeping a specified distance to thier targets
Not really, at least I don't see an easy way for that.
are destructable parts already included in spring-engine (for example just destroying the left leg of a k-bot/mech or the right trak of a tank) ?
Graphically, it's easy to represent: just use the two script command to hide and explode pieces. The gameplay effect of disabling the ability of the blown piece depends on what ability, but most of the time it's doable. Not firing a weapon is easy, halving the firerate of a weapon with two emit points is more fiddly, changing the max speed is easy, so is disabling cloak, radar, stealth... Of course most of it is non-standard script, so it's easy only as long as you understand how to write unit animation.
Finding out which part of your unit got his is doable, but afaik only me and maybe FLOZi have ever done it, even though it's just atan'ing the HitByWeapon arguments. You'll soon find it's not very precise though, unless you've huge units and slow weapons.
Targetting specific part of a unit is not very possible. Maybe one could rewrite the whole targetting system in Lua, or make mech composed of several units, or other hacks, but they'd take lots of work, lots of debugging, and have ton of side-effects that'll take years to iron out. So
in practice, not possible.
the emp/stunn could be used for heat, can weapons project emp to the unit firering them?
What? Do you mean can unit be paralysed with their own weapon? I think yes with the proper weapon TDF tags. But anyway you should use Lua Lua Lua Lua gadget Lua instead of making the unit firing a weapon on itself.
can u set the ai of a unit that i stops firering when emp reaches a critical level?
Easy to do, both in gadget and in unit animation. If EMP is just standard paralysis weapon, then it's done by engine. If you want your custom EMP, you'll need your custom code to keep track of EMP and call whatever custom function you want when it reaches whatever level you want. Not hard, but again, you need to know how to code basic stuff.
can u dice
Yes, there is a math.random(6) in Lua, and a Rand(1,6) in BOS.
events like ammo-explosion, reactor breach or pilot blackout when emp(heat) goes beyond the critical level?
Spring is not MechCommander engine, so it doesn't have events such as those hard coded. You will first to think about how you want to represent them in term of gameplay, graphic, etc... then how to implement them, then write the code yourself. Maybe it will just be three lines of code, but you'll still have to write them yourself, in a gadget or animation script. Spring is generic RTS engine, that started of by using the content of TA. Spring units don't have heat levels, don't have nuclear leak, don't have pilot, don't have ammo. At least not by default
So first ask yourself: Pilot passing out: How do you translate that into game terms: Do you make the whole screen goes black but otherwise everything function as normal? Do you make the unit stop moving and stop firing for 5 seconds? Do you make the unit carry on current order but not accept new order? Do you just tilt the pilot head (assuming you have a tiny pilot modelled and ringed in every mech)?
Then same question for the other events. Ask yourself what kind of impact and representation it has ingame, then find a compromise between what you want to do and what can realistically be done, then code it.