I do run an OS21Tm in my revo. For a t-maxx though, there are more options since you can run a side exhaust engine. Considering the cost to performance of the 21TM, unless you really really want the engine, I'd go with something else. If it was $100 cheaper, it would be worth it. As it is, $260 for the t-maxx version just isn't justified.
Which leaves you to either find a big block conversion or trying to find an alternate mid-block drop in, which are also getting harder and harder to find now that OFNA is out of the game.
In my searching, I find that LRP now has a pull-start
21 big block that would probably do really well in a conversion. Pretty sure all they had when I last checked was a bump start 21. Guessing it has less torque than the 28S3, 30S2, 30S4, 32S4, but probably higher RPM's and less fuel consumption... wish that was available when I bought my 21TM, I'd have given it a try instead since I have a spare new era big block revo mount. Less power than the 28 would be kinder to the drivetrain and easier on the fuel, but with LRP's longevity/build quality. Those qualities would also help with a t-maxx bb conversion since your still stuck running 1/10th scale diffs, cvd's and trans parts. Still probably would need to upgrade to RRP steel in the trans.
Back in the day when I had a 21 t-maxx conversion, I had a very hard time keeping diffs and cvd's alive with just an OS21RG which is a very tame engine by todays standards. But, now with revo spec diffs and slider axles, you'd be left dealing with only center axles (mainly the rear center) and trans gears being weak spots.
After doing more searching, I did find this:
Unlimited Engineering Mono-block t-maxx extension
Site says they have a dogbone for their chassis:
Extended dog bone
And perhaps their 6mm cups:
UE 6mm axle Cups
Not sure how/if you can fit that on a stock maxx with the extension and what drive cups you would need. You would have to sort out your own engine mounts and make your own holes for it as well as a pipe/header for a big block.
