I'm sorry, but it's not possible. To gain popular support, Bela Kun had to appeal to Magyar irredentism, by saying that he'd take back Transylvania, Slovakia and Croatia (From Yugoslavia). So they went for Slovakia, which was the easiest target. The Romanians knew they were next, so did a pre-emptive strike.
At that time, the situation was quite simple:
- at north, the new Czech state was quite isolated and weak
- at east, the frontline/demarcation line was along the Tisza river, natural defensive position and the romanian army was somehow strong (for the record, there were a shooting war between Romania and Hungary BEFORE Kun).
- at west, against the austrians the hungarians do not have any real interests/lands to get back, so at least, for the moment, everything put ad acta.
- at the south, the new Serb-Croat-Slavon kingdom was untouchable, because of the french troops stationed there (any attack against them means an instant war with france, suicide, especially since everyone on the hungarian side gambled upon the french non-intervention, at least, not an armed one).
After the northen campaign, the czech got really isolated and separated from Romannia (and thus, the Entente, supplies and such) and yes, the romanians would been next - and after the northen evacuation, the HRA launched an assault on the romanians, wich collapsed badly (since as earlier i mentioned, simply vanished along with morale).