The top one will and is fairly popular. The bottom one looks like it should, but note that there is no pull start attached...that means it is a bumpstart engine. Find the same one with pull start and you'll be much happier.
i ran the os 15 cvx and its a great engien if you dont want to fool around with needle settings all day to get it to run right, it may need a better cooling head but thats about it, befor i went .21 i got 11 gallons thrue mine befor i rebuilt it.
the 15 cvx also needs a crank cut but thats about it. it is a very reliable engine and will start every time
You will need to cut the output shaft until about three threads are showing. You will need to get longer header screws, as the stock ones are about 1/8 inch too short. You will need to get a stock collet to use for the flywheel as most aftermarket collets are not going to work on the output shaft. Other than those mods (which you would have to make with just about every engine out there), you should be good to go.
Others will recommend using either the OS rotary carb or TRX Pro .15 rotary carb in place of the Fantom carb. I can not vouch for that mod as my Fantom carb works just fine.
You could put alot of motors on your maxx just some require more modifacations than others. I would go with the OS but sometimes you might see that the pull starter is rather large and will be a tight fit. You could also go with the HPI .15ss or if you can find an HPI evolution.
I have the fantom and I have spent a decent amount of time tring to get it to idle properly it runs good and hauls balls but it does have a super picky carb. I may be tring the carb swap in the future. Still the motor has a lot of go juice. Lastly all this stuff about this excellent Customer Service. I haven't seen it. I sent an email over a week and a half ago still no reply also I tryed calling twice got machine both times. Lastly the parts are tough to find and EXPENSIVE. Anyway that my two cents on the Fantom I also have a OS in my GT and it was a brezze to tune but you will pay for the OS for sure. X
Almost forgot you want a motor that has some serious juice the 2.5 can put the stink down that puppy hauls balls. When my Fantom is running very warm it acts like a 2.5 running at 225. If the 2.5 is reliable that will be my next motor in the T-Maxx. X