Hi Hien, I can well understand you not wanting to pay the price of a new Honda engine - I've only bought second hand ones myself.
The Briggs 550 series are entry-level engines with aluminium cylinder bores. This means that they are capable of lasting a long time in home service provided you change the oil regularly and keep it topped up between changes, but they are not suited to commercial uses. The horizontal crankshaft engine is 127 cc OHV, which is well suited to your needs. (The vertical crankshaft version is 158 cc OHV.) Note however that the standard output shaft (which Briggs calls the PTO, or Power Take Off) is 5/8". You have said that the current side valve engine has a 3/4" PTO, so you would have to buy a new engine-side clutch half for $80. This is quite significant in proportion to the cost of the engine, which is $220 in the US. On the other hand, if your SB45 turns out to have the old captive cotter style of engine-side clutch half, there is a possibility it needs to be replaced anyway.
You have not posted pictures of your SB45. There are several things that might be wrong with it in addition to the possibility of wear/damage to the engine-side clutch half. I suggest you post a series of pictures including showing the engine mounting area, the cylinder reel, and the chains & sprockets inside the chain case on the left side of the mower. Deejay, our reel mower guy and the star of the Outdoorking videos on repairing SB45s, can then discuss it with you.
