There is a location for an internal 2.5' HDD/SSD in the S145 so you should be able to use you old HDD, but I do not know if the HDD bracket (P/N 5B40S21900) or the SATA cable (FS540 NBX0001NZ00)) were included with the purchase of the SSD model or not. Sometimes Lenovo will install the HDD bracket, but leave the cable out and include it in the box, but other times they just do not include them, so you may have to open up the laptop (see the links below) and check.
If they were not included, I do see both parts needed available from the COMPUPART SOLUTIONS's Ebay store https://www.ebay.com/usr/compupartssolutions?_trksid=p2047675.l2559
HDD Bracket
https://www.ebay.com/itm/5B40S21900-Lenovo-Hard-Drive-Caddy-81MV0001US-/174057618229
SATA Cable
https://www.ebay.com/itm/NBX0001NZ10-Lenovo-Hard-Drives-Cable-81MV0001US/153922354709?hash=item23d67cbe15:g:lTIAAOSwBGJetBKR
Note: If you plan to do the work yourself, be sure to follow the steps outlined in the documents/videos below closely, and be sure to unplug the battery immediately after opening the base cover, and avoid electrostatic discharges (static) that could damage your laptop.
Removal and Replacement Videos - IdeaPad S145 (81MU, 81MV, 81MW, 81MX,81ST, 81N3, 81N0) / Lenovo V14 (81YB) / Lenovo V15 (81YE) (this should apply to your 81S9 model too)
https://support.lenovo.com/us/en/solutions/HT508546
Lenovo IdeaPad S145 Series Hardware Maintainence Manual (see page 40)
https://download.lenovo.com/consumer/mobiles_pub/s145_series_hmm_201902.pdf
Cheers,