Rules

Here are some rules that I have tried (or am planning to try) for the period:

Pike and Shotte
Battles with model soldiers in the 16th and 17th centuries
Steve Morgan

One of the Black Powder family of rules, Pike and Shotte is similar to Black Powder but with enough period changes to make it different.  Some may find the treatment of pike and shot as separate units difficult to come to terms with but I have found they give a decent enough game.  They don't have any Montrose specific scenarios but do have army lists for Scots Royalists and Covenanters.

You can find an after action report of a game here.

For God King and Country
A Guidebook to the English Civil Wars
Bruce McFarlane

As with the other Canadian Wargamers Group books, this is a guide to the period with a set of rules and scenarios. The mechanisms seem to be a mash up of the melee rules from Flower of Chivalry (their medieval rules) and the musketry from Habitants & Highlanders (their French Indian rules) with a few specific rules added.

Of course it, like their other rules, does suffer from some rather poor proofreading (as you can see from the cover!) But as the book contains scenarios for Montrose (Tippermuir and Aberdeen) it's a pretty good place to start as it requires the least work anyway.

Bloody Kingdom
Wargame Rules for the Civil War in England, Ireland, Scotland and Wales
Stephen Danes

I believe this uses some of the same mecahnisms as Stephen Danes' Thirty Years War rules Father Tilly but with some specific Civil War elements added. Since I haven't yet played either set I can't yet voice an opinion but they look interesting based on a quick read through.

Bloody Kingdom doesn't have any pre made scenarios for Montrose; however, it does have sample army lists for Scots Royalists and Covenanters.