Im currently looking for a 2nd generation Acura TL and i keep finding ones with high mileage and i just wanted to know if it would be ok to upgrade the engine with 120k plus miles?

 

These are Honda Engines... @120K its barely broken in if it was well maintained you can upgrade it (Bolt-ons) to your heart content..
If well kept you can expect to put out a million miles on a Honda Engine without needing a rebuild, Although you will need to change those Timing Belts and Adjust valves along the way.

 

at 120kmiles you want to replace the timing belt and related parts

Adjust valves as noted above- critical to performance and general running- frequently overlooked/postponed job due to the high cost quoted by shops

install a lightweight crank pulley to lose 10 of 12 pounds! off the end of the rotating mass = way faster revs into vtec rpm
choices from 30 to 200 dollars for the part

trash the intake resonator box and install hi flow air filter

our mega moderator and master modder fsttyms1 (fast times won) pulled his engine at 200kmiles and dismantled- the cylinders still had honing marks = not even fully broken in!
Only removed engine to install a cl-s engine and 6 speed manual trans, car now has over 650,000 USA MILES on it and going stronger than ever~
He did 15kmile oil changes and kept up on maintenance

Your much bigger concern should be what trans will I replace the stock one with?
a used accord from 06-07 is a popular choice, and better than a rebuild on ours