It would be a big enterprise, although not impossible, Spain at the times of the Protectorate of Cromwell in England was a potence in total decadence, in 1643 in Rocroi, the Tercios had been defeated by the french in a symbolic bloody defeat that marked definitively the end of the supremacy of the Tercios as a military formation in the battle field of the Europe in XVII Century.
Spain was in bankrupt, the product of the mines of Potosi and New Spain, the gold and silver of America all had gone to fund the wars of Spain (or the luxuries of the noblemen), the politics of Olivares that in some moments seemed that could make Spain the supreme power of Europe although at the cost of having a supremacy a lo soviet (big military apparatus, with a economy simplily weaked and drained of their resources and men for the needs of the glory of Spain), finally even such supremacy was a lost dream, and the overextension of Spain proved fatal, with the state in bankrupt, the Tercios defeated, Castilla in the conditions of total poverty, Catalonia in rebellion, Portugal independent de facto (although the independence of Portugal not would be definitive until the treaty of Lisboa of 1668) and making a war against Spain, The Thirty Year Wars a disaster, the war against France a nightmare.
This was the Spain in times of Lord Cromwell Protectorate, a broken potence that was in the middle of a long way of decadence.
In these circumstances Cromwell naturally had had great resistances in New Spain against the possible conquest, but Spain was in no conditions of offer too much reinforcements to counterattack an invasion of New Spain.