If so, it is a Domoto 150 clone, so try Bonanza Dominicana (Sto.Dgo). You can try going to Bonanza in Santiago with some samples. Most of the parts will be the same, but not all.
Bonanza carries two 150cc scooter lines (both scooters look same) and those are Domoto 150 and Vento R4i also called Vento 150. Most parts are same (Domoto parts are cheaper though) some are not (very very similar but not same) and so you have double the chance your Loncin part will be either Domoto fit or Vento fit when you go to Bonanza. Have your old part at hand.
Also, good place to buy parts for these scooters is in San Cristobal. these 150 cc look-like scooters are plentiful there, as well as parts.
You may also use this trick: ask for parts for DOMOTO 150 parts and not loncin, when you look for parts. Also it's always to have the old part/sample at hand because most "repuestos" buy from various chinese vendors, most parts are universal, but there are differences, so have the sample for exact fit.
Believe me, "domoto cientocincuenta" is a well-known phrase among repuestos in the DR.
P.S. There is also a big brand (japanese sounding, but do not recall the name) scooter rep in DR, they have many spare parts for 125/150 scooters. They have a huge warehouse just by Carrefour in SD. The same intersection you go to Carrefour (coming frm SD) you turn right but do not enter the CF parking lot but rather take the road uphill, there is a big warehouse in a curve.