I would greatly advise to go with a Millenium overhaul by a reputable shop rather than a 'factory' rebuild.
I have 600 hours on my Millenium cylinders with zero problems on my current TCM IO520.
My friend just installed a Superior IO550 with 5 year warranty (condition to oil analysis). It runs very smooth and powerful. Zero problems during or after installation. Fuel flow was set right too, not too lean as most factory engines. Too lean full rich fuel flow will damage the engine during the first hour.
The previous engine in his previous airplane was a nightmare, new Topcare cylinders lasted no more than 250 hours in two years despite very careful engine management as recorded by JPI. Warranty claims were laughed away by TCM rep.
Look at the crankshaft disaster by LYC.
If you have a good running engine, keep it and have it rebuilt by a local reputable shop, they will stand behind their product.
Saving money will come later, by greatly reduced maintenance bills and properly honoured warranty.