I'm not an expert on the Revo by any means, but I'll share some big block conversion experience and things I've read about it. I think you can still get the Traxxas big block kit for the Revo, but I'm not sure which engines would work without modification. The New Era mount is the best one that was made for this kind of mod and is able to fit most any big block engine, but I don't know if it's made anymore. I'd be very tempted to try this if I had a Revo using a Dynamite Big Red .28 or an LRP .28 or .30. The only issue is if it doesn't fit, you are likely stuck doing a bunch of cutting, grinding, and some custom fab work to make it all fit. The usual issue with such a modification seems to be the crank and size of the case itself as well as mounting position in relation to everything around it. I ran into the crank issue with my big block Maxx, had to cut a few mm off the crank end and shim the hell out of the clutch bell and motor mounts to make the gears line up and have the crank not hit the transmission.

I have seen some with the LRP engines in them, but I think I remember hearing you have to grind the engine mount from the kit out to accommodate the large crank case. Also, a pull start doesn't seem to fit to well using the Traxxas kit. It might work with a roto start set up, though. You'd need a proper exhaust as well, so you'll have to rig up a side mount pipe so you can use an 1/8 scale buggy system. If you want to do the big block, I'd try and find the New Era mount. Don't forget the drive train, either. The extreme power of a true big block will rip the stock drive train to shreds. I know all about that, you'll have to change all your drive shafts to steel CVD shafts, get a 1/8 scale clutch, and shim your diffs for sure. You might end up having to upgrade your diffs, too. I run stock brushless E Maxx diffs properly shimmed in my T and so far they're holding up.

If you decide to go drop in, I'd have to say the OS .21TM is probably the best overall option. A friend of mine I run with often has one is his T Maxx using a THS T Maxx exhaust and it's an excellent performer. The OS engine will tune much easier and hold a tune better. It is also much stronger than the stock 3.3 ever could be. OS engines cost more, but they run for a long time and just run great overall. The only engine I've ever ran that tunes as easily as an OS is my LRP. Another option would be the Picco P Max .21 which is a direct drop in as well. I'm not sure how good it is though and it's the same cost as the OS .21 is.