There are many ways to convert an OL to a monster. You don't really need a suspension lift, you could just do an RCM shock lift. Which is basically taking sheets of styrene cutting them out and adding new screw holes to it. You then screw it onto the original shock mount and screw the shocks into the new holes at the bottom of the styrene, this would then "lift" your OL up. There's another method of taking a $5 toy called "Xtreme Rock Crawler" or something like that from Walmart or KBtoys. The shocks are longer than the stock shocks so they'll also lift your OL up. If you can't find those long shocks there's a member at MiniZRacer.com that's sells them in his kit. His name is WarnOffRoad and I think his kits consists of 4 shocks, a tow thing to put up front, 4 tires that a bit larger than stock tires, and I think it's either a little roof rack or a light bar. Just to warn you it may be a little hard to get a hold of him as I don't see him online often. You could always try e-mailing.
A simple body raise can be acheived by placing the body right between the shock mounts and the battery holders. Another way is to take a material like lexan and create a new "wall" on top of the chassis and add velcro to it and the underside of the body.
It depends what you call "monster tires". There are tires called Super Swampers that are larger than regular tires and are excellant for rock crawling. However these are no longer in production but there is a member on MiniZRacer.com by the name of RCMotosport(aka RCM). He molds tires that are basically a replica of the Super Swampers. His tires are called Super U's and they require almost any
wide racer rims. I think he's also experimenting with new molds too. Another definition for "monster tires" would be like the tires seen on the MiniZ MadForce Monster. If that's what you mean then Tamiya has a model kit with tires similar to that. You can take those tires and I think with minor modification you'll be able to run them on an OL. Of course a body lift would be needed. Here's a little picture gallery by arch2b that has the tamiya monster tires:
Click Me. He has a bit of a description under each pic which sort of makes it a little tutorial... if it's not clear enough you could always search the Overland forum at the minizracer for more in-depth instructions.
For more information you can visit the Overland section of the MiniZRacer Forums.
PS - If I got anything wrong please correct me as I'm just playing this by memory